for the past few weeks i have been putting in the volume with 15-20 hour weeks. I started at around a 19bmi and now I am officially classified as "underweight" by bmi standards. I have been eating more than ever though which is really strange -- about 3500-4000 calories every day. I make sure I am never hungry. Performance on the bike has just been absolutely incredible. I am not fatigued too much ever and been really just riding so much better, especially on the climbs (probably since I have dropped 4-5lbs).
I feel totally fine but got a physical examine next month and i am a bit worried the doc is gonna say i am underweight and freak out my parents if i am eating enough and not trying to lose weight or starving myself to increase w/kg (which i am not), and my riding is getting better with higher volume, is there anything too honestly be worried about besides some calculation saying i am "underweight"? I am sure my low rhr (38-40bpm) will be worrisome to the doc and can't imagine my blood pressure being high too.
Firstly, if you're that concerned i would examine what you typically eat on a normal day. Break down the Marcos (Fat, Carbs, Protein, Calories). Then, how are you training? How many calories are you burning that day? If you're burning more than you take in, you're generally going to lose weight. It's as simple as that. Unless you have an underlying issue physically, I don't see any other cause.
There's nothing wrong with being slightly underweight.
If you get bloodwork done and all of that, and if your vitamin and mineral levels are good, and everything else is good ... be happy with slightly underweight.
As long as everything else is good ... you're simply burning more calories than you're consuming. That's how weightloss works. Enjoy it while you can because, unfortunately, as we get older we often tend to exercise less and tend to put on weight.