I used to be able to have sex 3 or 4 times a day now after the first time i either can't get an erection or lose it. i am 28 should i be worried?

Refractory Period: This is normal. The "refractory period" is the time between climax and obtaining another erection. This interval gradually increases with age and is quite variable between individuals. A 17 year-old may have a refractory period of 10 minutes (or even zero), while a 70-year old man may take several days to be able to have another erection.