No energy....

Ok so I been off birth control for about three months now, I'm overweight but have lost alot and still am, anyway! My last period was 6 days late then I got it for 2 days, very heavy and then left, I'm never late and it never last just 2 days, so I have been super tired like can barely move let alone do anything, I took 2 test both say I'm not pregnant, if I'm not pregnant what else could be making me feel like this.