Since you have not taken any injection in the past five months, it is unlikely that your current symptoms or the negative HPTs are due to the depo shots taken earlier. As natural ovulation is likely to have returned, you should get an estimation of the serum beta-hCG
titres to confirm/exclude pregnancy
as this is very sensitive and specific. You should also get a trans-vaginal sonogram to clarify the situation. Hope you find this information useful. Wish you good health.