The good news is you are no longer on Bactrim. I am happy to hear you are off of it; it's awefull. It remains in your system and makes you sun sensitive so be careful. Blood in the urine is a concern.
It sounds like they are sending you in the right direction with the urologist and nephrologist. That is great news. Keep fighting and ask questions. Ask them what the blood is from if it is not from what they tell you they "thought" it was. I would ask for a 24 hour urine test; I've done them in the past and are no big deal. In fact, they should have already asked for that along with a GFR, checked for casts, etc. Lupus sometimes does effect the kidneys and it's called Lupus Nepheritis. Check out a the book from the library called Lupus for patients and families by Dr. Daniel Wallace. You are going to need to be your own advocate and push, push, push. Don't take "no" for an answer. If you have to ask for a second opinion; do so.
I will keep you in my thoughts and prayers. Stay positive and keep pushing for answers. Let us know how you are doing.