Hello.
After ejaculating, your partner should hold on to the base of the condom firmly and remove his penis from your vagina. That will keep any sperm from dribbling out. There are no guarantees in life, and accidents happen, so you should always be sure to use lots of spermicide and the aforementioned technique in order to minimize your risk. If you're worried about pregnancy, you can either get Plan B over the counter at your pharmacy, or get an early pregnancy test. Dr. J
Once a man ejaculates in the condom, it is wise to pull out right away. Otherwise, you are increasing the risk of breaking the condom.
You state you felt it trickle down your butt crack and outside of the vagina (while he was still inside you). Most likely, none did not get inside you but there is a possibility you could get pregnant. You did not state he pulled out with sperm leaking out the tip of the condom so that's good news!
The spermicide only helps kill the sperm on contact but it's not 100% effective.
Based on what I read, I don't think you got pregnant so calm down!
In the future, tell your boyfriend to pull out IMMEDIATELY after he ejaculate into the condom.