You should probably ask a healthcare professional. However it might well be somewhat normal for a person that hasn't been doing high cardio activity regularly. Took me most of a year back in my early 50's to be able to ride hard without getting too much complaints from my body and weird stuff happening. 10 miles back then was a big effort and took a lot out of me. 13 years later 50 miles isn't anything..
If your doctor says you are good to go, then just keep going. As your body gets stronger and use to it you'll see the weird issues go away and you'll feel better in general. Though you'll probably just get another set of weird issues as you progress. <grin>
Just finished a 42 mile ride and might have to go walk with my spouse. So eventually things get better.