I am not certain in your circumstance - but I suspect that like armor, damaged weapons start losing strength. A place to check first anyhow.
When armor is damaged you can see the value drop. At level 7 I would not worry about repairing, you replace stuff pretty quick at levels below 20. Or craft one to hold you over.
Weapon power is a stat which boost the damage you do with any given physical weapon in your hand. The damage stat given on the weapon is specific to that weapon.
It is very likely that you changed a piece of armor that affected it in some way IE buffing weapon power directly or possibly Stamina buffs its output as it does with stamina based skills.
Is it possible at all though for your weaponpower to be lower than your weapon damage? Isn't weapon damage a number that gets added to weapon power by default?