Not pregnant

So I'm young 19 almost 20 and trying to conceive for about 3 months I have not had a menstural cycle for about 3 months and I use to have them all the time like several times a month for like 5 or 7 days however everytime I take a pregnancy test its negative any suggestions why? I was on the depo shot for about 1 year and got off of it in November of 2015 so almost a year now other then that I have not had any changes and I have had several periods after getting off the depo shot. any suggestions would be great thanks!