What Causes Excess Weight Gain While On Depo Shot?

Hi, i was on the contraceptive pill Marvelon without ever having any problems, i then changed to the depo injection after i had an abortion after falling pregnant whilst taking marvelon, but i gained far too much weight on the depo injection, i then got put on to cerazette mini-pill, it was terrible the bleeding, clotting and cramps was that bad i went up to A&E because i thought i was having a miscarriage so i then stopped cerazette and went back to marvelon, i am now bleeding whilst taking the pills and not just on the pill free days, the bleeding goes from brown to red and back to brown and i am very worried i never had any problems before on marvelon or is it because i have just changed over from cerazette please help, I started bleeding day 4 of this pack i have 7 pills left in this pack should i have 7 pill free days at the end of the pack or should i carry on to the next one and also is this normal??? Please give me some answers