I went without a period for 4 or 5 months and it all came down to stress I also had started working out (nothing extreme either). With your past problems that could be it too since your changing up stuff like eating and everything. It's frustrating but the doc had told me everything was normal and to stop worrying because that was just making the stress worse which was messing with my cycle. You might get checked out just in case something else is going on but more than likely it's stress.
I am going through the same thing right now. I am 26 years old and I have two kids my periods are irregular also. but for almost two months now I haven't gotten my period yet and I also took a few pregnancy tests and they all come back negative.
This happened to me a couple of months ago too. I missed my period for two months str8, then it came the next month. It could be that your cycle is changing. My period was due a couple of days ago and it hasn't come yet. But my periods always been somewhat irregular since my first one. I hope this helps you a little.