hello there... welcome to healthcaremagic.. well i want to make one thing clear that if u have got periods either more or less.. pregnancy is completely ruled out.. means you wil not get pregnant at any cost that too after having heavy bleeding in between.. so dont worry about it.. you are not pregnant.. as far as heavy bleeding is concerned it may be caused by a hormonal imbalance and in hormaonal imbalance somtimes you may get periods earlier and sometimes later.. and the lower back pains and stomach pain may be due to hormonal imbalance or some other gynaecological problem.. i advice you to get consulted to a gynaecologist if the pain continues.. and yeah remember.. you are not pregnant....if u are still not sure then get a pregnany test kit like velocit which is available over the counter in medicine shops and get a pregnancy test done at your home using an early morning urine sample,.. i hope this solves your query.. i wish you a speedy recovery and good health.. thanks & take care..