This sounds serious, you should see a doctor right away. It may be a complication of intestinal infection, hard to say.
It sounds like some type of arthritis. Some of the autoimmune conditions can affect you stomach also. I get the bruises and black-and-blue marks also and I have a lot of joint pain from arthritis (they say unknown type, osteoarthritis in my knees).
I recently read that Salmonela posoning can lead to arthritis condition. I am not an expert but only researching because I am researching my own condition. It started off like a head cold, went to "stomach flu" not vomiting just diarhea. The diarrhea has held on. I've had it for 2 weeks. So, trying to research how to alleviate it as I have no medical.