I was wearing splint mail. and I've got AC of 2 by that. and I put on a shield. and It become 3?
I had 2 slots on sword and shield.(I had long sword) why does it Increase, instead of decrease?

Wow. lots of new users in the boards.

Are you sure you didn't have 2 slots in single weapon style?

Also, could a temporary spell effect have worn off (eg, defensive harmony)?

I was in irenicus's dungeon, so no defensive spells. and I had two slots in single.

Well there's your answer. IIRC, 2 slots in single weapon style gives you a +2 AC bonus; so when you equipped a shield, your AC went down 2 because you were no longer using a single weapon, but you regained 1 because of the shield; net AC goes down by 1. Solution: unequip the shield.

another qustion:If i use magical shield, does it improve my AC?

A shield +2 would (AC bonus 3). Effectively, you're getting a +2 AC bonus from the weapon style (as well as other bonuses), so you'd have to recoup more than this for a shield to be effective.

The Senital sheild which you can buy in Ust Natha gives you +5 to AC. That is the best you could do AC wise in SoA.
You can easily get sheilds giving you AC+3 in Chapter 2.

However, it is a major waste of proficiency points to put them in single weapon style and then use a shield...unless its for specific situations, such as using the Shield of Balduran while going Beholder Hunting...

Also, putting points in Shield and Weapon style is a waste as well, as it only gives you a paltry bonus to AC against missile attacks...which you mostly dont need once you get a decent shield.

Just allocate those extra points to different weapons.