Hey there,
i noticed that sometimes my stamina is regenerating very slowly...
It's just tired, OP. Give your stamina a minute to rest. You wouldn't want it to stumble from muscle fatigue and take an arrow to the knee, would you?

But seriously, like the person above me said, the rate of Stamina regen is extremely variable depending on what you are doing.
Standing still = Normal Stamina regen
Walking = slightly slower regen
Running (like from a Dragon) but not using sprint = effectively half of the normal Stamina regen rate
In combat (of any type) = Stamina regen slows to a crawl
There are many magical effects, diseases, and enchantments that affect the above lists as well, so even these can be countered/altered with the right enchantments. If it was a Frost Dragon that you were fighting, having some potions of Stamina and Stamina Regen Boosting would be an effective way to counter the effects of the Dragons frosty breath. Plus the effect of Ice will further slow your Stamina regen on top of having it drained.
Going into a battle prepared will help you face little to no major set-backs on most battles that you will face. In the advent of a surprise attack (Dragons...), just having a few vials of potions like this will often help get you through without getting your ass handed to you.
I wish you luck in your future battles, but go visit the local apothecary. You'll be glad you did when you get into your next major fight.
