1) AP's can cause you to stop having periods.
2) Each woman reacts to each individual AP differently
3) Due to (1) and (2), it is possible that you stopped on Risperdal and resumed on Seroquel.
If you are still on Risperdal, then I do not know how Seroquel would do that to you.

At any rate, report it to the p-doc, and if the p-doc advises to see a gynecologist or a midwife or a family physician, then do that.