15 weeks and bleeding tissue?

Hi all. Hoping someone has had a similar experience. I've been spotting multiple times a day for 6 weeks now. Sometimes light, sometimes a bit heavier. I've seen my doctor multiple times and have had three ultrasounds, the last one being a few days ago. They find a heartbeat and say baby is growing on track every time. Christmas eve late at night I bled a tiny bit of tissue for the first time. Then again Christmas morning. Now this morning I'm bleeding tissue again but it's quite a bit more. I have a fetal doppler and have been able to find a heartbeat. I called the on call doctor this morning and they told me the same thing I've been hearing for weeks, nothing they can do. Just take it easy and wait.

Anyone else have something similar happen? I understand nothing can be done but I'd like to at least know what's the cause or if it's going to be okay.