What Causes Delay In Period?

Hi,My last period was December 17th and till date i didnt get my period.so last week i went to my gynecologist. and they told me i am pregnant but it is weak. and he gave me 10 days medicines also.but yesterday i went for my checkup and he told me that there is nothing.that mean i am not pregnant and he gave me gynaset
px norethister one tablets IP 5g. could anybody help me that why and how its happens? I am still confuse that i am pregnant or not? and please guide me that now what should i do? (My English is very poor but please help me)