I’ve been getting the same issue, where my stomach crunches and my brain says I didn’t have enough food when I got all my nutrients. That’s because Huel is nutrient dense - it’s easy to carry, but really confuses our brain and body that are used to a larger volume for the same nutrient content. So I eat carrots sometimes to fill my stomach and that stops my hunger pangs and satisfies my brain. I even get a little energized from that too.
I think you might need to let your body either shrink or get used to the new food to get energized from it. It’s like a reset - as your body is tuned to your previous diet. To change my diet, my body takes from 3-4 months to catch up - which may be different for others. Hope that helps.
Also, I would give your body rest if it’s going to transition. It’s not going to be able to think straight (i.e. adjust) if you’re putting it under stress. So I would slow the exercise, much as your body’s slowing down to adjust and then when you start to feel better, then you can bring your exercise back up.
This is from a person who got first place for biking for the women’s in a triathlon round and 6th place in a turkey trot among other races.