ur wife is probably suffering from Low Blood Pressure(Hypotension) in which these symptoms occur. also find out that she might be creating this condition herself to avoid intercourse? build trust and understanding. ask her, do she experience these feelings in the day time also? see, backache may occur due to generalized weakness and hypotension. take care of her diet and rest. Ask her to take dinner properly and drink milk or glucose at bed time.
after intercourse, some amount of semen do comes out, its normal. don't worry.