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It could just be muscle spasms. Get a test and see. If it comes negative and you still have doubts, wait a few weeks and get one done at the docs :)
Definitely get a test done. Because HCG (the hormone that the 'pee-on-a-stick' tests measure) declines after a while, go straight to your doctor and ask them to test. With Depo, you're probably aware that you need to get a shot every three months (unless you're on some other form that lasts longer, but I haven't heard of one). If you got that first one and nothing else, you might be five months along. Good luck with everything.