My pubic area and penis is swollen after lipo. Why did this happen?

Thank you for your question and I am sorry to hear about your swelling.  Often as a result of the fluid placed in the abdomen to safely perform liposuction the surrounding tissues will become swollen.  In addition, as you become more active with walking after surgery gravity will have a tendency of pulling this extra fluid downward, causing swelling of your pubic region and penis.  I would speak to your surgeon about care for the area, but using a compression garment, as well as time, will improve the issue.  

No matter what type of lipo is done, when it is performed on the lower abdomen and under local anesthesia, which I assume you had, this can be normal. 
The anesthetic fluid runs out through your lower incision but unfortunately some of the fluid can bypass the incision and  continue "south" and end up swelling your pubic area, penis and testicles. 
Usually, as you describe, it is pain free but somewhat frightening at first. Your body will absorb this excess fluid in time, usually a matter of days. 
Again, as long as there is no pain, redness or warmth in these tissues, it is normal. If these other signs appear, call your surgeon immediately. 
Also make sure that your garment is not too constrictive where your penis and testicles exit. This could significantly delay the resolution of this swelling.
Gentle massage, ie. lymphatic massage can help, also when lying down, elevate your pelvis with a pillow.