LH Surge

Why did my LH surge only last a short amount of time? I tested with opk this morning and it was negative, tested again at 130 pm and I got a positive digital opk, then tested tonight with LH test strips and another digital opk and it was negative. We didn’t have sex until tonight after seeing negative opk. Did I miss my surge? Will I still have good chances of pregnancy since we didn’t have sex until tonight?