It sounds like Fortify Health where your health is temporarily boosted thanks to enchantments/effects of equipment and potions
Fortify Health is an enchantment and an alchemical effect that temporarily increases the maximum health for a limited time.
This would explain why your health is boosted but then after a while it drops. Salvaged/bought Fortify Health Potions normally have the effect last for 60 seconds.
There is a Blessing of Arkay which
Increases your health by 25 points.
Which would explain it and your health is dropping by 25 points because the blessing has worn off. if you keep an eye on the time you should notice the change after eight real-time hours of gameplay. if you check the Active Effects category of your Spells Menu you should see "Blessing of Arkay" there