I can't tell you what it is, but I can tell you what it probably isn't, and that's cancer. I worked as an educator in a reproductive health care facility -we did gyn check ups and lots of out patient sterilizations (for women and men), and we saw all kinds of reasons for spotting and cramping. The vast majority were not serious. Some required treatment, some didn't. Sometimes we didn't find the cause until the doctor was doing the surgery for the tubal ligations, and he was able to see the problem with the laparoscope (especially fibroids were easy to see). That was a few years back, and the diagnostic techniques are much more improved since then.
I haven't had polyps or fibroids, but I've certainly had plenty of mid cycle bleeding, and I had a lot of tests and treatments during my infertility. I've also had some abnormal pap smears, mostly because of an insufficient number of cells, and I have a past history of breast cysts and questionable mammograms (not the same problem as you, I know, but the worrying and waiting is the same). I've gotten rid of all those problems but I still remember what it felt like to be anxious and without answers.
Yes, they are annoying as hell, and it is frustrating when you don't have answers right away. But you have plenty of diagnostic techniques available to you, and if you don't feel your GP has the expertise, then ask for a referral (which she will likely give you anyway). Plenty of good and highly trained professionals can read your ultrasound or other tests.
If it's a hormonal issue causing spotting (which happens all the time), it's usually not hard to fix. There could be a short-term use of medication (including but not limited to a low dose birth control pill), for example, that can set you right.
Sometimes a hormonal issue can mess up ovulation or cause a cyst (which are benign), and there can be some mid cycle bleeding. There can also be cramping. It's absolutely not serious.
The likelihood of cancer is so low, it's really not on her list of concerns, I imagine.
I know you don't want anyone to tell you to calm down and not fret, because that doesn't help anyway, and if you tend to be a worrywart anyway, it sometimes causes more stress than people hope they are relieving.
I work in food science now and spend a ton of time with women who have menstrual, menopausal, or hormonal issues, including PCOS and irregular cycles, infertility and UTIs, heavy cramping and hot flashes, so I see it all on a regular basis. There's so much you can do in a non-medical approach too. So once you decide on how you want to proceed, there are lots of options available to you that can include medical intervention or not. The main thing is to seek answers without the panic keeping you up at night. I know that's easy for me to say and really hard for you to hear!
Hang in there.