I also have had fybrocistic lumps in my breasts, which I first noticed 20 years ago. It actually led to my having a biopsy in my early 30s and beginning to have periodic mammograms around that time. And that's how my cancer was found because it is of a type that does not form distinct lumps (ironic, really). I'm just pointing out that these things can have unintended benefits--I don't know that I would have been as faithful with the mammograms if it hadn't been for that original lump.
But what you are asking is whether you should trust your doctors or your own instincts and I think there are a LOT of women on these boards who will tell you to go with your instincts. It may be nothing but if you feel like something is not quite right you need to persist until someone takes you seriously rather than dismissing your concerns. I suggest making an appointment with one of your doctors when you know the lumps will be swollen. If you still can't get them to check it out, go to a different doctor. If it is nothing to worry about (and it well may be) then you will have peace of mind and that is well worth the hassle. Good luck!