Well it could make you weak just like playing basketball for 12 hours straight or doing house chores all day would wear you out or make you weak. Sex is a physical activity, unless it's her lying lifeless and you're done in 2 min. Then, not so much.
But sweetie...everytime you have sex with anyone, how ever many times, you increase your chances of disease or pregnancy. Are you using protection? Are you both tested? If I were you, I would check with a local free clinic about contraception, contracted diseases, and get yourself tested if you've both been with other people before each other. Educate yourself before you EVER engage in any sexual activity, THIS INCLUDES oral sex.
If you were both virgins to each other, or have been tested, then you shouldn't have anything to worry about. But do remember to use contraception unless you both are planning on having children.