How common is suture migration?

Thank you for sharing your question, what you’re describing is something that is not at all uncommon for many patients, regardless of the procedure they undergo. 
Dissolvable sutures placed deep to the skin closure will often “spit” or become exposed, possibly due to their location or a person’s response to the foreign material in his or her body. Many times the consequences of this are minimal, with irritation of the skin or the suture pulling on clothing being commonly reported issues. 
Show your Plastic Surgeon the areas of concern and see what he or she recommends. Often the suture can simply be cut at or below the surface of the skin to solve this problem. 
I hope this is helpful, and glad to hear you’re happy with your results so far! 
Peter A. Felice, MD

Hi and welcome to our forum!
Suture spitting is quite common when deep sutures are utilized. Most deep sutures are of the dissolvable type. However, on occasion, an individual may be hypersensitive to the material utilized or the suture may be placed too close to the skin surface. The body tends to extrude foreign materials unless biologically inert. Older dissolvables were composed of animal tissue and were destroyed over time by the body's inflammatory reaction. Spitting sutures were more common back then. Newer dissolvables are synthetic and dissolve naturally without inflammation. Spitting still occurs but to a lesser extent. 
Treatment consists of removal of the offending suture followed by a short period of wound care. It is an annoyance and an inconvenience for the patient but fortunately is usually short lived.  If this phenomenon occurs in multiple places along the entire suture line, it may be wise to take note of the type of suture utilized so it can be avoided in future surgeries.
Continue followup with your plastic surgeon.
Best wishes...