Having trouble conceiving second child

My husband and I were incredibly lucky to conceive our first child immediately upon trying. We have a gorgeous baby boy who is now 16 months old and we would love to have another baby. We’ve tried 3 months in a row now but no luck. Im scheduled to get my period in two days and the test is coming up negative. I know it hasn’t been long but my cycles are extremely short. 21 days to be exact. Does anyone know what could be going on? Love some help!