Spotting?

I've been diagnosed with endo a few months ago. As treatment we've began with birthcontrol, I'm on my 5th version by now because the previous attempts didn't work for me.

I'm now 2 months into taking my new pills, without stopping to get my period. I've been taking them non stop, and lately I've noticed I've been having a lot of brown discharge. There's nothing else wrong, no yeast infection, no smells, no itching. I've spotted before with the other pills but then it was clear it was blood.

I've read online that it's probably old blood, but I'm not sure. Do you think it is? I've been having it for a week and a half now.