By now I hope you've seen a doctor about this, but for anyone else with the same question I'll leave this answer.
I have Crohns disease (a form of IBS) and had this same problem with both constipation and diarrea 6 years ago. I didn't have a bulge above my naval but did have a bulge in my buttocks. I first asked my pharmacist what he would recommend for a case of simultaneous constipation and diarrea, which he replied was "impossible". Aside from being unhelpful it made me mad because it's obviously possible if I'm asking him for help for it.
I went to my gastroenterologist (GI) doctor who informed me that it was an abcess (internal swelled mass of infection) that hasd formed in my buttocks around the perianal tube. He immediatly had me admitted to the hospital for emergency surgery to have the abcess surgically drained. It turned out the abcess was form by fistulas that are small canal-ways that are common with crohns disease. They generally start their formation at the area specific to your crohns outbreak travel through the body cavity to somewhere else where they drain intestinal matter into the connecting tissue. With me it connected around my anus in my buttocks and created an abcess infection there. My father 30 years ago had the same thing but it connected to his kidneys (very serious.) Where they end up is up to them in there own unfortunate way.
In regards to the question at hand, having constipation, diarrea, and a bulge for someone with crohns I'm of course reminded of my situation and want to help anyone else who may come searching with the same problem.
Things to note though; the constipation and diarrea in my case was caused because the abcess "bulge" was in my buttocks so as the abcess enlarged it pressured against the anal canal squeezeing it smaller and smaller making it difficult to even pass diarrea hence the constipation, now in this case with the bulge being above the naval an abcess may have formed and is pressing against the local part of the intestines making it difficult to pass matter through the digestive even with diarrrea. I'm not a doctor and you really should see one if you have these symptoms, but the MOST important thing to look out for if you are reading this and have a similar problem is FEVER. If you have an internal infection you WILL start to develop a fever, at this point it is SERIOUS and you should go to the emergency room and tell them you suspect you have an abcess. An xray will confirm any abnormal swelling that could be an abcess and they will take it serious from there.
I hope this helps someone, although preferably I hope no one ever needs this help. My prayers will be with you and all who have to deal with Crohns as I have had to most my life.