I wonder (new on this),
Someone absorbed my strength and the effect seems to be permanent (after days playing, I'm not fully sure, but I think is still lower that usual). Is that possible? Is there a way to reverse it?
Thanks.

Try using any of the Restore Attribute spells or potions.

attributes/skills can be drained/absorbed (temporary) or damaged and the later is permanent, well... at least until you use some restoring effect or seek help in the cathedrals. Not all damages can be recovered in all cathedrals and you shall find spells or (I think) potions to correct the issue. the spell have specific names. restore strength (for example) and the amount they can recover, never surpassing the max or the "homemade" that have an amount over time.

Remember some diseases may cause attribute loss too, and so you would need a remove disease effect (indeed diseases can be cured at most temples over there).

Edit: Not uncommon is attribute lost permanently by using the console command SetAV "attribute" # while wearing items that increases this same attribute. To these only console command again will help.

Actually I did lose 15 points of agility, and other some for strength ( i think I was bitten by a spider). The numbers are not in red, but I figure by comparing with old saves. I applied a poison cure poison, disease cure poison, restoring agility poison, but it doesn't help. Why is that? and please, how can I restore what I had? rising levels is pretty slow in Oblivion.

if it isn't red the attribute isn't counted as lost. Are you certain it wasn't green before? implying you had some modifier in it?
Another possibility is you having mod that changes the leveling system. They tend to recalculate attributes and skills by level. If you feel cheated by the game you can aways use the console with the command: ModAV attribute/skill value --- where this value is a modifier to be "added" to the attribute's "base value", and even can be negative if you care the final result doesn't falls below zero.

Edit: beyond modifying attributes and skills, ModAV can be used to change the secondary ones and several other stats too, like health, fatigue, magika and so on.