What Causes Missed Periods With Negative Pregnancy Test?

I've had sex with my partner and i do have irregular periods but this time it is 3months passed. last month i donated blood during blood donation camp. i did my urine test and the result is negative. but now since i havent got my periods my mom wants to take me to the doctor. I'm scared. What shall i do? will the doctor come to know that i've have sex?