What?!

[SwiftCore]

When I reached level three, and got the feat weapon finesse, for a while my attack bonus went from +2 to +11 (possibly +13, I was very caught up in the moment of having so much AB). This, of course, is quite unusual as I only have 20 dex. Shortly there after, it fell down to +8, and +9 with a +2 rapier. Why in the world would this be? How do I get my abnormally high AB back?

[Jasimine]

When I reached level three, and got the feat weapon finesse, for a while my attack bonus went from +2 to +11 (possibly +13, I was very caught up in the moment of having so much AB). This, of course, is quite unusual as I only have 20 dex. Shortly there after, it fell down to +8, and +9 with a +2 rapier. Why in the world would this be? How do I get my abnormally high AB back?

The attack bonus in your inventory screen shows were your attack chain left off the last time you were swinging in combat. Since your attack chain adds bonus's to your swing as you move further into it it is completely feasible to see 5, 10, and 15 point jumps.

Also when moving you get a -4 to hit penelty, which is also depicted in your invintory when you move.

In order to get your standing attack bonus add the following yourself

Base Attack Bonus
Relevent Stat Bonus
weapon bonus
feat bonus' (weapon focus, greater weapon focus, etc)
buffs (prayer, aid, greater heroism, and the like)
Enhancement Bonus'

Another option is to simply log out, wait 10 or so minutes for your character to completely reset, then before you get into a fight and while standing still open your inventory. This will show you your unbuffed bonus with your weapon.