I was 23, when I had my first bleed between periods. I didn't think anything of it but then it happened everytime. I have been to the doctor and had lots of different tests and was told that it's just the way I am. I'm now 28 and have had enough.
It's rare for me not to bleed between periods. It can range from a dot of blood to bleeding for 2 days. It usually happens during ovulation time but can be any day between periods and can happen more than once. I am much more likely to bleed when I am stressed (which is often) and done physical activity (which isn't that much exercise). I also bleed a great deal when I have done instense physical activity, done any level of activity when I'm already tired and whenever I'm VERY excitied (like when I've met a singer I like or sexually excited with my man).
I get tired period pains and feel emotional, hormonal feeling, when I do bleed. I am a virgin and have regular periods which are very heavy and last 9 days.
Well, just from the title alone, I was going to tell you: bloody discharge between periods should always be taken seriously, so get to a doctor. Then I read that you have already been and had lots of tests. I assume that things such as endometriosis and uterine polyps were ruled out, and your thyroid has also been checked already?
If all serious causes of bleeding between periods were ruled out, then you should know that abnormal vaginal bleeding CAN be idiopathic, meaning no medical cause is ever found. If you think your doctor neglected to check everything and you are very worried, do ask for a second opinion though!!!