Could I get pregnant??

Me and my boyfriend have been pretty good about using protection. But last night we did not. I started my period on Sat and it usually lasts 4-5 days. I thought it was finished yesterday, but after we had sex, I went to wipe and there was light pink on the toilet paper. The glow app says I'm 75% fertile 😳 I'm thinking I should take Plan B?? Are the chances really that high?