Just a quick coffee/pregnancy question?

for the first 6+ months of being pregnant i was really good and cut out caffeine,but i am 29 weeks and cannot tell you what a difference it makes in my energy level to have just ONE cup of coffee, how often do you think is ok?? i had one saturday, monday and having a teeny tiny cup today.........i feel so guilty about it but at this point in pregnancy my body is getting very tired!!