I had three weeks of pink discharge from 28 weeks on. Like you, I was back and forth to the hospital to be checked (and given another dose of anti-D, told all was fine but to come back if it happened again. It was intermittent for three weeks, and after my third visit I pushed a bit harder as to whether I really should keep going back when it basically wasn't stopping, but I had no other symptoms and they kept finding nothing wrong. In response to that, midwife said that they prefer to be cautious, she didn't want to tell me not to bother, and I should come back if I was at all worried - but "if it was me I wouldn't bother for pink discharge". Basically, they didn't want to tell me it was fine, although it probably was, and it was up to me how safe to play it.
Because it was just pink discharge, not proper bleeding, because everything else has been normal and fine, and all checks/scans they did looked fine, I didn't go back. I've had a midwife appointment in that time and all checks were fine. I mentioned it to her and she agreed with the "probably fine" advice - although stressed I should call in if it became proper red blood, I had any pain, or was at all worried about anything. Everything has continued normal and it's pretty much tapered off to nothing this week (am now just on 32 weeks).
It's worrying, especially when they don't find out why it's happening, but it can be one of those things. As I say, mine was very minimal pink discharge, all movement was normal, and no pain or other symptoms, so after several checks I felt safe riding it out. But I had to push a bit for them to confirm that wasn't foolhardy! Don't ever be afraid to go get checked if you're worried - but don't panic. I was told it can be nothing - to quote "It's not normal but it is fairly common".