Is it possible for a seroma to develop 6-8 months after Tummy Tuck surgery, no drains?

It is possible you have a seroma, but it is very uncommon this amount of time after surgery.  An ultrasound can show whether your lump is fluid or not.  The first treatment, if it is a seroma, is aspiration and compression.  Please see your surgeon again and kindly ask for an ultrasound.

I have not seen this happen, but this could be diagnosed with needle aspiration and if negative, then an ultrasound to see if the lump is solid or fluid-filled. If fluid it makes sense to treat it sooner rather than later, so it does not become a bursa.  If a bursa forms, then a non-surgical treatment that has  been used injecting it with tetracycline solution.  That can cause it to collapse.  If that were ineffective, surgery would be needed to remove it, which perhaps could be done through the umbilicus. It is good to avoid seromas which drainless tummy tucks are very good at preventing, since treatment involves one or more of the steps above.Drainless tummy  tucks are an integral part of my procedure.  Even with drains or tissue glue there is a a 5-45 % incidence of seroma per the literature. A meta-analysis in the Aesthetic Surgery Journal, 

Aesthet Surg J. 2017 Mar 1;37(3):316-323. doi: 10.1093/asj/sjw192.

Seretis K, Goulis, D, Demiri EC, Lykoudis, EG

Prevention of Seroma Formation Following Abdominoplasty: A Systematic Review and Meta-Analysis

reviewed all the literature on this topic, and found that "preventive measures" decreased the incidence of seroma four-fold. One of the studies included in the "preventive group" that used only progressive tension sutures, from 2006, showed fluid accumulation between the sutures, detected only on ultrasound. This fluid was not clinically detectable by palpation or inspection and resulted in no need for seroma aspiration or any complication. The total amount of fluid associated with clinical complications including drainage in the other groups was greater than 80 cc, and the total amount of non-clinically detectable fluid in the progressive-tension group was always 80 cc or less. The use of drains plus progressive tension sutures did not change the amount of fluid, so there is really no benefit to using drains plus progressive tension sutures.

The study was discussed by Pollock and Pollock, who noted that in their series of 565 patients using progressive tension sutures they never had a clinically significant seroma. When a seroma occurs, it may require multiple visits and aspirations, and if persistent, introduction of antibiotic sclerosing solutions, or reoperation to remove a fluid filled fibrous cavity. The drain opening may leave a scar in the incision or pubic mons which may become pigmented or depressed. Not having to use drains spares the patient and plastic surgeon from the chance of these undesirable events and/or outcomes.

My experience mirrors that of Drs. Pollack.  I have had no clinically significant seromas while using progressive tension sutures in the past five years or so.
Rather than use the 15-20 individually-placed sutures described by Drs. Pollock, I completely close the dead space with continuous PDO and monoderm Quill sutures. These sutures effectively reduce the dead space to almost nothing, so even the small amount of fluid that might collect between the sutures, cannot create any significant seroma. Using running Quill sutures allows relatively rapid dead space closure as no knot-tying is needed. The umbilical location is confirmed with a Lockwood flap marker before the lower abdominal closure is completed.

While most seromas are discovered shortly after the drains are removed and there is an obvious fluid wave there can be seromas form later if there is trauma to the area or hematomas. This could have been a small seroma not recognized early on but developed a thick bursa and then contracted into a more round hard mass just now being discovered. Yes an US can help diagnose a seroma bursa and differentiate from a hernia that you would not want to stick with a needle.