At times doctors just dont know what the answer is, so they guess. The best thing to do is that you need to keep at it and find out what the problem is for sure, not just them thinking it is this or that.
For endo, the only way for them to tell is from a Lap. I just had one done two months ago, and found out I do have endo. I wasnt suprised. Just so you know you can still have kids even though you have endo. A lot of women have endo. Also the surgery (lapo) it cleans you out and also you are more likly to get pregnant after you have it.
I also have IBS lol ya I have a few issues that took a long time to find out what they where. For me when I have pain from IBS its more on my upper right side. And when I get my period is the only time I have pain from the Endo. Hope that helps a little bit.
Request for more tests. Have you had your intestines looked at? might want to chat that out first.
Goodluck