every body has variable sex capacity. one thing I would like to tell you that " excess of everything is bad" . you are sexually active and quite potent but you are still looking for more sex. now think in the way I am telling you , suppose you have capacity of eating 250 gmail of rice and if you are forced to eat double or triple then you would feel uncomfortable. similarly if you would ejaculate then again you would notice that penis is not that tight and it loses erection and similarly when you need rest after being exhausted your penis also needs some rest . take a break allow your penis to regain its strength then only you can again enjoy sex. this is normal you do not need any medicine. just reassurance and counselling of both partners would be beneficial.