Not a doctor.
My understanding is that PEP is just a strong dose PrEP. Common HIV medicine includes
TDF/FTC (Tenofovir Disoproxil Fumarate/Emtricitabine, e.g. Truvada)
TAF/FTC (Tenofovir Alafenamide/Emtricitabine, e.g. Descovy)
Raltegravir (e.g. Isentress)
Dolutegravir (e.g. Tivicay)
For PrEP you take either Truvada or Descovy usually. For PEP, you are taking at least two (such as Truvada + Isentress) at higher doses.
If you have been taking PrEP every day and now switch to PEP, I think you should be fine. PrEP, when taking as directed, should prevent the vast majority of HIV infections, and switching to PEP, in theory it should only offer you more protection cos it's more pills. However, if prior to your unprotected sex last Sunday you were not taking PrEP as directed, taking PEP is not a "I can do whatever I want within these 28 days"--it only serves as a last resort for protection for that one time of unprotected sex.
Even if you were infected on Sunday, it would be unlikely for you to be infectious within a week.