Loan interest that is not consistent.

Am writing this because I have tried questioning same and the consultant failed to answer my questions and I keep paying more. I have asked before I think somewhere 2017 as to why is it that my interest charges is not consistent in that in February I can be charged more and march get charged less interest. I was told you charge 28% interest per annum but the way things are I think am being robed. I want answers why the interest I am being charged sometimes goes up and sometime goes down. My understanding is that when I make payment and the outstanding balance goes down the interest also goes down. Why does this one goes up sometime.