Kissed boyfriend, potentially exposed ?

It can take up to 10 days to start getting symptoms.

Rather than questioning the cause, go get tested and make sure to inform your mother and anyone else you’ve come into contact with 

Did you give it to your BF?

I've read reports saying that symptoms can show up anywhere from 2 days to 14 days afterward, but the average time is 4 days, and most of the time, it takes less than 7 days. That's why the CDC now says to self-isolate for 7 days after you've been exposed. So yes, you got it from your boyfriend. I hope you're isolating until you're over it. 

If he's got it then you probably have too.  You should stay at home and keep away from anyone who isn't already infected, especially if they're old and sick.  From your story you're probably young enough that you're not in any serious danger, you should recover okay buy you may still get quite ill.  I've had it and it was quite nasty, but I'm probably twice your age.

You could have gotten it at a grocery store or anywhere even if you wear a mask. Most people don't realize they touch the inside of their mask when putting it on or straightening it out on the face and if you touched a surface with covid on it you will transfer it to the mask that touches your mouth. No telling where you got it but you should get tested and then self isolate for the recommended amount of time. If your young and healthy it should just be like a cold for you but you could potentially kill and older person or someone with diabetes or other health problems. 

It's not clear to me; does Mom have COVID? If she was infectious a week ago, wouldn't she be sick herself by now?

There's no way to be sure which of them infected you, if they both have it. You need to let your mother know you were definitely exposed by your boyfriend and that you're not feeling well now and very likely are infected.

And you need to stay the hell home, isolated completely. If you live with others, in your room nearly all the time, masked when you leave it. I know, it's a pretty miserable way to have to live when you don't feel good, but if you infect others, it's going to eventually be passed to someone likely not to survive. Can't have that.

Yes you have been exposed and if he can actively transmit covid you're going to get it