Bit concerned. Sore tummy.

My stomach is feeling very sore and I don't know why. Feels like I've been punched really hard just below my left boob in my tummy. A bit like the pain you get when you've done too many tummy crunches. Having difficulty getting up from my seat because of the pain. I'm 9 weeks post op. Have started driving this week (long story) and upping my activity including lifting (was told to take it easy for 6 to 8 weeks after 2nd open surgery).
Had 2/3s of Waitrose low cal chicken casserole for dinner. Pain came on bad after eating. What do you think I've done? Pain isn't unbareable but uncomfortable. Bit worried.