Yes, you could be pregnant. The "pull-out" method is VERY unreliable. Precum can contain sperm.
Sore breasts, nausea, and cramping are possible signs.
Normally you would ovulate between about day 11 and day 16 of your cycle. The egg is viable for about 48 hours. Having sex after day 18 therefore, it is unlikely that you'd get pregnant - NOT impossible.
You had sex on day 17. There is a chance. Irregular periods make it very hard to calculte.
You can take a home pregnancy test as early as two weeks from when you had sex OR when your period is due. You must use your first morning urine, your wake up pee, for the best accuracy. Use a name brand test too. If it is negative and you still haven't had your period by about 10 days AFTER your expected period date, then see your doctor.
NOTE: false negatives are VERY common when you test early. False positives are VERY rare. The home tests do NOT always work and are really only accurate when you are pregnant - the default is "NOT" pregnant.
Hope it helps. Good luck.