make sure you're drinking enough water. if you are even slightly dehydrated it can cause the uterus to contract, and it can feel like a charlie horse! your urine should be a pale yellow or even seem clear if you are well hydrated.
as far as timing goes... i can't necessarily agree that this is fool proof. 1st baby - they were 5 min. apart for 3 days - yes... 3 days! (i was dehydrated...) 2nd baby, text book labor, they got progressively closer and stronger. 3rd baby, my contractions were kind of nagging in the morning, but by 5 pm they were strong, very strong, and never got closer than 10-12 min. apart. in fact, during delivery when they have you push with a contraction, i had to wait 10 minutes for the next push because they still were not closer than 10 minutes apart.
in the end, if you are unsure, go to the hosp. the worst that will happen is they will put you on the monitor to see how strong and close your contractions are... they'll give you an internal exam and if you are not dilated to 4 or 5 cm (this varies at different hospitals) they will send you home. no harm no foul. dont worry about feeling stupid... they see this every day.
i can tell you though, since this is your first baby, you are not very likely to have a short labor... you'll have plenty of time to go thru all the things that distinguish real from false labor.
congrats on your little boy! enjoy this time in your life :)
good luck and get lots of rest!