Vicodin/Percocet Inquiry?

Ok so my doc had me on Vicodin for a few months due to my intense back pain from being in multiple car accdts...I'd had enough of them and switched to percocet and just took my 1st one today. (I've had Percs before). Now I was off both for about 3-4 days with hideous withdrawals..just took the Perc today and its been about 3 hrs since I took it and I don't feel a thing, when I know I should be feeling it..I'm wondering if this is because I was off all meds and its just taking a little longer to hit my system?