Why can’t it be my turn again..

I am sick of this.. every month.. same ol indent lines or stark white tests. I am 31.. so I feel like my clock is running out.. I have three kids already, but I want one more. What if my body won’t allow me to have another baby? I had a chemical 7 months ago.. and ever since then, my periods have been irregular. I haven’t even gotten my period last month or this month.. idk what’s going on.. but it’s driving me crazy. I am over all healthy, other than I’m slightly underweight.. I’m 103 and 5’4.. so, what is going on!?!?