Seromas generally occur sooner after surgery than 15 week. It may just becoming apparent now. If you do, in fact, have a seroma, you should follow up with your surgeon and have it drained. This far out from surgery, a seromacath - which is a catheter that gets inserted and left in to drain seromas - may be needed. If the seromacath done not get rid of the seroma, then you may need a surgical procedure to have the seroma washed out with the scar capsule that forms around it removed, and then reclosed to allow it to heal.
Dr. Gary Breslow has 16 Tummytuck before & afters: