Any ideas of what can mimic pregnancy?

Ok, so for the last two months, all I've done is spot instead of my regular period. Last month i even had some odd, on and off spotting at the wrong time. As of today (06/06/11) I will be six days late on my period for this month. Since some time in mid april, i have had some serious mood swings, aversion to food and smells i've never had before, back aches, nausea, vomiting, fatigue, restlessness and a couple other early symptoms I saw my mother and other friends/family members experience in early pregnancy. I took five home tests, three came up positive. I went to the free clinic near my home and they sent me for a qualitative blood test at the local "hospital" (it's little more than an out patient center for physical therapy and a small emergency room with a lab). The blood test also came up negative. At 19 and about 3 years from even considering wanting a family of my own, I practically skipped out of the lab office when they told me that. Despite my total confidence in not being pregnant, all my symptoms have persisted and i've even gained a little weight in spite of the fact that I eat small amounts of food and make sure the food is (usually) healthy. The fact that I have crohn's disease also determines what I eat and I have a hard time gaining weight normally because of the gastrointestinal immune disorder. I also know it's not a "hysterical pregnancy" because that would require me to worry badly enough or want to be pregnant badly enough to convince my body that I am. I'm concerned something else is wrong, but my boyfriend is still really worried that I might be pregnant. So, anyone have any ideas??