Please, definitely go and see a different doctor - a woman one. They have far better understanding.
Whilst in my mid thirties I had exactly the same symptoms as you have described. I plucked up enough courage after 1.5 years to go the doctor and discuss - described the problems and she referred me straight away to the hospital.
The hospital I went to used a teeny fibre optic camera and had a look inside and found a polyp lurking in a fallopian tube.
They booked me back in as day patient two weeks later for a General Anaesthetic and extracted it. (Consultant sounded disappointed when she described as no bigger than a small fingernail!). I had no problems recovering from the op itself - very small amount of "period pain" for a couple of hours - I had more trouble recovering from the GA - that took me 3 days to get over. (But I smoked then as well)
I have had no re-occurance of either the bleeding since and this was some 8 years ago.
Please go and get it checked - most of the time it may be something minor/mild that is easier dealt with now.
I have also found that over the years leaving illness such as things like this or things like dodgy teeth unresolved have a draining effect on your health - low level, very subtle but draining none the less.