There are a lot of things which could be relevant:
1) Posture.
1A) Your usual posture is bad, so it's hardly surprising you're having back pain.
1B) You changed your usual posture on the bike so you're working new muscles.
2) Blood sugar/pressure.
2A) You're suggesting some memory loss from just yesterday, which is unusual.
2B) You say that you're heavy, so you could have mild diabetes or hypoglycemia which this level of exercise could exacerbate.
3) General fitness.
3A) You're out of shape, so of course you're sore.
3B) You're out of shape, so of course it's easy for your electrolytes to get off balance.
That's why you're getting so many different responses. If you'd like to keep this sort of thing up (which is great!), see a good sports doc about how to baby your body until it's ready for this kind of stress.