Covenant

Vow نذر when you commit to do a worship like fasting or praying recommended Prayers like Night prayer, or good act like helping needy or give something in good cause if you get your wish. 

Oath يمين is when you take oath to do something good or leave a bad deed like taking oath to leave smoking or leave attending mixed gatherings etc.

Covenant عهد is when you promise Allah (SWT) to do something good or leave something bad.
Fulfillment of vow, oath and covenant is obligatory.

If you don't remember what intention you had, you need to just fulfill what you had committed no matter what was whether vow or oath or covenant as all of them share a commitment which must be fulfilled. Failing to fulfill the commitment is a sinful act.

Wassalam.

'Ahd عهد (Convenant) can be to commit do something good or to commit not to do something bad.

'Wassalam.