Any advice?

I am a mother of two beautiful boys ages 5 & 3. My husband and I really want four kids so we've been trying to have another one. It's been over a year and still nothing and during this time my period has changed completely. I used to be regular my cycles were always 28 days. Now my cycles are 40 days and getting longer with each cycle. Like this month I'm on cycle day 41 and still no period and still negative pregnancy tests. I have been cramping and having a lot of back pain so I'm assuming my period will be here soon maybe. I just don't understand, when i was trying for my second child it only took 3 weeks after stopping birth control to get a positive pregnancy test. Now it's taking forever. The doctor has me on medroxyprogesterone. I've used it for two cycles now and still no regular periods again or positive pregnancy tests. The only thing that has changed is my increased back pain, I'm more tired and I had bled for like 20 days after my first cycle on it. My Doctor only told me it was a normal side effect. I don't know what i could do to increase my chances of conceiving, is it possible to become infertil after already having kids? I should also mention that i got diagnosed with hashimotos thyroiditis last year, but i don't take any medication for that, apparently it's not bad enough or something.