Way back in 2006-2009 I commuted 4500 miles in 10 months.
19 miles AM, 9 miles Pm was my standard day.
I was in my 50ś and roughy 10-15 pounds overweight.
Enough about me.
You are a young dude in ok-ish shape.
Your hilly 3 miles ride, how long does it take?
Unless your hilly commute has monster hills I would not expect you to be exhausted.
Do you have a primery care doctor or nurse practitioner?
I suggest you see your healthcare provider.
Tell them what you told us.
A comprehensive blood panel would rule out any unidentified health conditions.
I apologize if this comes off as harsh.
I mean no offense.
fat biker