You are only 14 and in my opinion, way to young to be having sex. The pull out method is not good protection against pregnancy. I can't tell you what to do but, I will recommend using a condom every single time with every single sex act to protect against an unplanned pregnancy and STD's.
Now, about your short period. Several things can cause that in a young girl. You should go ahead and take a pregnancy test. It's kind of soon so you might want to get a very accurate one.
Okay, breathe!
Now then...you had the breakthrough bleeding first? Then you had sex? Then you had your period which was short?
Your chances are 50% because you used no protection. The other 50% comes from the fact that you had a period after that.
If you had a period say on March 15th ( or around there ) and then you had another period on April 15th, then I would doubt that you are pregnant.
At your age your periods can still be a little mixed up.
Even at my age ( and I'm old ) my periods can get mixed up. Some months they are longer than others, and some months they don't come at all...it's all part of being a woman.
If your period was February 15th and then you had sex and then you had some spotting and then no period...then that would be a different story. I would be concerned that you are.
HOWEVER...I strongly suggest buying a home pregnancy test or seeing a doctor before you panic even more than you are.
Don't sweat okay? If it turns out you are pregnant, then just tell your mom...she'll understand.
Better yet, tell you mom what happened now, and she can get you on the pill. Don't be afraid. Your mom might yell, but she isn't going to kill you. She's been there herself...she knows and understands better than you think.
I've been there, done that. My mom didn't kill me...she yelled and asked how I could do this...and then she settled down half an hour later and started calling everyone to tell them that she was going to be a grandma. lol
Anyway, if you tell your mom now, and it turns out that you ARE pregnant, she can get you some help.
If you tell your mom now and you aren't pregnant, then she can get you on birth control pills before you do get pregnant.
Worry about your body and not your mom's feelings. Okay?
She'll love you no matter what.
If you need to, make a doctor's appointment, get tested, and if it's positive have your doctor call your mom, or bring your mom in so he/she can tell her.