What Causes Absence Of Period With Negative Pregnancy?

Hello,
Hope your are doing very well.We are planning to have a child sometime soon.My current problem is that I got my last periods on 9 th of march.I missed it in the month of april.And today it is 5th of may and i still did not get it.I have taken 4 HPTs ,one just 2 days back all came negetive.
I showed to a doctor 4 days back and he took a sonography and said i am not pregnant but said i have litttle bulky uterus.
I am worried because I am cleless,No periods,no pregnancy.I still wonder if the test results were not false.I want to conceive but with no periods coming,I do not know what to do.
I sometimes feel breast burning,tight stomach,infrequent nausea etc.
Also the doctor gave a medicine saying if I take itit wont affect if i am pregnant ,or if i am not it willrestore my pregnancy.I have not taken it yet.I want it to happen naturally.till date my periods have always been regular.
looking forward to your assitance.
thanks,
madhu