I am 2 1/2 weeks post OP for a umbilical hernia repair with mesh. I have a lump and huge rash by incisions. Have an appt tomorrow. Any ideas?

Reaction . You may have a reaction to the mesh or sutures. Wait till you see your surgeon tomorrow to get a definitive diagnosis. Good Luck .
Post surgery. You likely have local irritation to the stitches. Less likely, you may have reaction to the mesh itself.
Postop. It's good that you have an appointment tomorrow. The lump may be a collection of blood and serum(hopefully not puss) at the operation area, but the rash is of concern. Infection,manifested by redness(cellulitis), is a possible postop complication. The rash could also be an allergic response to something that came in contact with the skin.