How did u determine when to take a test

My periods are irregular....My last one was in April.....For this past week my cervix has been changing quit often one day it was low and firm and then the next day it was very high and soft....My nipples are sore to the touch and im sleepy....I know some pms symptoms could also be early pregnancy....But im not sure when i should test.....Would a cheap test help me determine if i am ?