Second pregnancy???

Hey ladies , so I had a my first baby a year ago . I have not planned on having another baby AT ALL ! I'm 18 years old and my baby Girl obviously wasn't even planned but was a blessing in disguise . After I had her my period was a little off but came back to normal . I haven't had sex since around beginning of June and like normal I had my period ( I think it was my period ) well I haven't had a period since . I wasn't really worried about it I've been going through alot of stress so I assumed it might be late because of that . Well I was supposed to start almost 2 weeks ago and haven't . I've been super sick on and off diarrhea like with my daughter but no throwing up and my arreolas aren't darker than normal which they were when I had my daughter . Other than that I've had the same symptoms like with my first pregnancy . Frequent peeing , fatigue , lack of sleep , stomach ache etc . I'm very scared I'm pregnant again but I don't understand how if I had my period last month I could be pregnant ??? I know this seems silly asking a bunch of women I've never met before , however with having a one year old and only being 18 I don't have a bunch of money to spend on a pregnancy test. Do yall think I could maybe be sick? Or should I suck it up and buy a test?