If you consume more calories than you burn, you will gain weight.
If you consume fewer calories than you burn, you will lose weight.
You do not state your cardio (type, intensity), but, doing some simple stuff, that you, yourself, can research....
1.5 * 800 cardio 1200
Workout resistance 600
Muscle mass 1781.25
Base caloric load 3581.25
You either not eating 3581 calories, or,
you are burning more than 3581 calories, or,
you are are doing more activity, or,
you have a a metabolic problem, or,
you're very ill, or,
you've got an eating disorder.
Common sense 101. These are all things you should be able to figure out on your own, and are easily researched at this site and others.
What are you doing wrong? Hard telling, but, those are most of the possibilities.
If you are so thin on 6000 calories, I'd sure look at a metabolic disorder. Almost anyone can gain with that many calories. It's important you know. You could have some other illness, too. Or, maybe you're not eating as much as you say.
If you have do 90 minute of cardio, as you said, and you're eating that many calories, you should still be fine.
Eat a jar of peanut butter every day. See what happens.
If what you're saying is true...I'm guessing you have a hyper-thyroid condition. Everything I know about it, and you, suggests that. Either, that, or you aren't counting something right.
Just FYI, if you think for a moment, you'll realize that your post workout weight loss is simply water loss from sweating, and metabolic process. That's very normal, and very typical. Again, common sense 101.