how do you know if your pregnant or if its just pms?

i just wanna know how to know . i feel so lost. my period is so random and irregular amd i just feel like im completely clueless. im not really trying but my periods late. i had sex with my ex around 6 days before ovularion and my periods late but this isnt the first time its been late and its meant nothing ... i just dont know what to go off on and if i should test. i really just wanna wait because im sick of buying tests because they are expensive and them bein negative