I returned to work as RCA, * or * days per week after ** years retirement CSRS. I call...

I returned to work as RCA, * or * days per week after ** years retirement CSRS. I called OPM and emailed OPM to make sure this would not affect my pension and was assured it would not. The last ** weeks I have been paid $.***hour instead of $**.**. I was shown a formula that took the difference between my annual pension and my projected **** income assuming i worked full time (**** hours) and then divided the balance by my hourly earnings, $**.**. The grand total was $.***hour. How can this be? I only work *** days per week. I was assured I could make $**,*** annually before **% of my wages would be kept. Why can't I work part-time? It's hard for my postmaster to find someone dependable that only wants to work *** days per week. Why are they basing my projected income as full time?**employee id ********