First I'd like to say welcome to our forum. I certainly hope that you don't have lupus, but if you do, you'll find lots of great people here to help support you and give you information.
To answer your question as to if you can lupus w/o a positive ANA, yes it does happen. It's not common, and it's I believe only in 10 to 15% of the cases. Usually, a person will have at least one positive ANA and then it goes negative. At least that's my case, I've had like 4 positive ANA in the past 18 years, but I normally test negative.
As to the hem/oncologists I'm sorry that they want to put you through a bone marrow biopsy. I've have 4 done, I have, in addition to my lupus, a rare blood cancer called MDS. Which I stand at 35-40% chance of changing into full blown leukemia. Your oncologist is worried about your low WBC and that's probably why he's wanting to the BMB. I hope that everything turns out alright and it's all normal. Hopefully, it's just the medication that you are taking that's causing your WBC to be low.
Gastro problems can be common in us lupus patients, but there are other things that can be causing your issues. I hope that your colonoscopy goes well and that get the results to you quickly and that they are good. The only advice I can offer to you is to keep a diary of all you symptoms and take pictures of any rashes. Try not to let yourself get worked up to much over the BMB, I won't lie they aren't pleasant. However, each person is different how they react to them, we have a member here who is 14 and she had one done. There are several others who have had them too. Each of us can comment differently on it. Like I said I've had 4 of them done, it hurt for me, but it was what I needed to be done. It's certainly answered some important questions that were being asked. Please try to eat something, just some little something. Know that we're here for you, if I can answer anything else give me a holler. Good luck and let us know what happens.
dx fibro, SLE, glaucoma, cateracts, bells palsy, depression, migraine headaches, gastreopaersis, chronic anemia, MDS (Blood Cancer). Mediport and tons of meds.