I understand that you are anxious, but there is no reason to be worried. The blood spotting and light period is due to the effect of the oral contraceptive used. When you are on the oral contraceptive pills they generally cause a hormonal imbalance that results in spotting between periods. This should gradually subside and your menstrual cycle should become normal. There is also some amount of weight gain seen associated with the use of oral contraceptive pills which probably could be the cause of your weight gain.

If the bleeding doesn't subside after the next menstrual cycle, then you could consult your doctor and she could help you with it. In regard to the last part of your question, the pregnancy test that you had taken was after 4 months and your pregnancy test came as negative that indicates that there has been no ovulation and with no symptoms suggesting otherwise. I hope I was able to clear your queries.