It usually says somewhere - inventory screen or stats screen - what modifiers you have. If you see any, mouse over them to find out what they do (if they don't already state it).

Apart from that, are you wearing any heavy armour? Hungry (though hunger status is one of the more obvious ones)? Maybe poisoned by something?

I restarted the game and now all is back to normal. Looks like a bug or a feature, where I was having the effects of hunger but then when I consumed some food and was at 0 hunger the -1 modifier was still lingering...

Were you carrying too much gear? That can affect your AP, although I'm not sure it chnges any stats.
Was your character suffering from either Poison or Radiation sickness?
Did you eat/use something that lessened some of your stats while raising others?

Those are the most common things that can modify your stats, assuming you weren't maxed out on hunger. If it happened after you rested or traveled a long distance in a cut-scene, smoetimes you can get super hungry and lose a couple of health in addition to the normal effects of hunger.

I got that too. No statuses in stats screen nor inventory but modifiers still apllied. Its a bug I think. I just saved and reloaded game and the game was back to normal.