Working out and gaining weight! Help!

Weight loss is going to come mainly from moderating the calories you consume rather than the calories you burn. Are you tracking and measuring intake accurately for weight loss (based on TDEE or the goal that MFP sets for you)?

If yes, sometimes starting a new workout routine can cause weight gain due to water retention, but it should be temporary as long as you are consistent with the new routine.

This is me right now. I am eating well and definitely staying within my calories, however I am working out harder than I ever have in my life. The scale has gone down and back up but I feel a difference in my body and my mood. I know I am retaining water because I am feeling muscles I didn't even know I had. You didn't say how long you have been back "at it." If you are comfortable to say that you are logging food accurately, I'd say you need to keep with it and give your body time to adjust. I made sure to take measurements as well. Check them every couple weeks and you'll probably see a difference even if the number on the scale isn't dropping like you'd hoped.