I can underatand you are concerned and shall try my best to help. The presence of brown spotting
even between your cycle along with the fact that you have heavy bleeding during your period does raise suspicion fir a few conditions, let me list them for you:
-Abnormalities with the inner lining of the uterus
-Soon approaching menopause
-Trauma (often seen in women trying to conceive, rare; but will have to be ruled out)
And a few other causes.
I would recommend a bimanual examination, speculum examination, pap smear
, hormonal level check, transvaginal ultrasound
scan and a urine analysis. These investigations are all simple but will surely help in finding the cause.
Hope this helps.