What Causes Stoppage In Periods?

i have recently come off the depo and was due to have another one 1st april but didnt want tro go back.. since then i hvent had a period but the past week my breast shave felt very heavy and sore... ive felt ill on and off the past 3 weeks due to bugs and stuff.. ive taken 3 pregnancy tests all being negative i was wondering if these are signs coming off the depo or because im due a period after such a long time....?im concerned it could be somethign serious?