Hi aimze, sorry to hear about your recent loss
If you've hit negative, then it's unlikely to be GTD cells.
Mine admittedly did get pretty low before they shot up again, but nowhere near as low as yours, I also needed chemo, like m2c did. Mine was a pmp.
Also mine took forever to drop down, unlike yours, 16 weeks I think before they went back up. Chemo blasted it tho!
We started trying 6m after the chemo (shorter wait time due to age) fell pregnant on the 12th cycle, but miscarried, then went on clomid and got our son as a result! We have low count (mot/morph acceptable) i have a luteal phase defect and my egg quality is declining, so this time round TTC we hit it hard from the start, had two early mc (one I didn't even know about, but charing cross did!) and then this pregnancy on my 6th and final cycle of clomid!
Good luck for this new bfp, I reckon it will be most likely a pregnancy. Have you tested again? Are the lines getting darker? Fingers crossed!