I havent gotten my period in 4 months. by the end of the 4th month i had intercourse, its been 6 days from the day and i am now spotting . what is it?

Several things: The first question is why u haven't had a period.If u have done pregnancy tests and they were negative,thisis likely due to not ovulating which can have many causes. U need a full history, an exam, and appropriate testing to know which is the cause. If u aren't ovulating the lining can get so thick that it starts to fall off leading to spotting or prolonged bleeding.U need to be seen ASAP. Be well