Fitness questions

Some background: I was a bookish child, did nothing much in the way of sports and reached adulthood without having acquired much at all in terms of muscles. I’ve always been very flexible though. As an adult, I spent too much time sitting in a chair at work, which gave me 2 ruptured discs before 30. I tried occasionally to “do sports” in order to strengthen my back and generally my body. This time round (I’m 38 now), I’ve been doing weights exercises at home, which I’ve really enjoyed. However, I always come up against the following issues, and I need help understanding if I’m doing something wrong, or if this is just approaching old age, or if there is something wrong with me (which – I hope not!). I’m female, btw.

1) I seem to lose any acquired muscle within days. I worked my way up to doing a 90s plank, was doing squats etc. and I was able to take the stairs at work without breathing hard. Yay! Then I got sick, had to pause for a week, and I’m wheezing up the stairs like I’ve never done any sport at all. Is this normal?? Why are these muscles degenerating that fast? It took me weeks to build them!

2) I’m sore, a lot. If I try to prevent the issue above by doing my exercises every second day, I’m just sore all the time. If I wait until I’m not sore, I feel like I run into issue 1) again.

3) maybe the same as issue 2), but I stretch in the evening, to get the tightness out of the back of my legs, for instance, and in the morning they are tighter than ever.

This has always been the case for me, when I went running and when I did martial arts as well.

I’m frustrated and annoyed – why, body?
Can you help me better understand if these issues are normal? Am I doing something wrong? Are you experiencing these issues, too?
Thanks in advance!