Could be a subchorionic heamatoma (sch)
I have one now and conceived for a second time after my 3rd ivf and 1 MMC.
It was only after I read up about them that I found out they are apparently more common after ivf.
In my case I hemorrhaged out of nowhere at 11+6 weeks and assumed the worst, but a scan the next day showed happy baby growing on track.
I bled and spotted for 5-6 weeks before it stopped, but my sch was quite large.
The sch is thought to be a tear between the uterus and the chorion. Most often they think it originates from the placenta attaching but not always.
The majority heal eventually, but it can be a bit nerve wracking while you are waiting for it to happen.
The medics tend to be fairly hands off with respect to these, mainly because they can't do anything about them... Just expect you to watch and wait, but there are lots of support groups online that you can find if you google.
BTW, I am now 23+6 weeks with my ivf baby. The sch is still there but stopped bleeding about 6 weeks ago and is slightly smaller than it was. Hoping it will either completely reabsorb or stay as it is for the remainder of the pregnancy without causing any further issues.
The typical characteristics of an sch bleed/hemorrhage is a sudden bright red gush which can contain clots but generally isn't accompanied by much cramping and slows fairly quickly. Bit like turning a tap on and then turning it off again soon after.
Only a scan will diagnose this if you have any blood pooled inside, but thought it might help you to know that this could be a possible cause of the bleed which doesn't necessarily mean miscarriage.