Was the diagnosis specific? Mixed connective tissue disease, undifferentiated connective tissue disease?
These are complex diseases and often include lupus symptoms as well as symptoms of other AI diseases. If your kidneys are affected which is often the case with lupus you could be experiencing edema and weight gain. It will be very different from normal weight gains and if easily identified in your feet, ankles and lower legs. I gained as much as 20lbs of water weight in just a few days but a combination of meds took care of it and I now take an ace inhibitor to keep the kidney condition in check.
You brief description sounds like you might have some GI problems and possibly kidney involvement. When I was sick my rheumy ordered many tests to determine the extent of my disease and I saw a gastroenterologist and a nephrologist due to various symptoms.
Here is the Lupus Foundation site wtih a description of lupus.
MCTD or UCTD are more complicated but here is a site to help you understand what overlapping connective tissue diseases are.
It is important to find a rheumy who is experienced in AI diseases. Think about getting a second opinion.
MCTD (lupus, scleroderma, polymyositis). Diagnosed 2005. Kidney, liver, GI tract, dysphagia, raynauds, Barretts esophagus, quadriplegic in 2005. Recovered and now active in skiing, tai, chi, hiking, golf. Meds: prednisone 2.5mg, imuran 25mg, amlodipine, benazapril, omeprazole, potassium, folic acid, vitamins, maxide and various supplements and vitamins.
Remain optimistic and you can overcome the odds.