Pregnant? One week early periods 3 times in a row and the last one lasted only 2 days

For the last 3 months my periods have come 5-7 days earlier than the time before. The duration was 5 days each time as usual and I bled as I normally do, but last time my period arrived, another 5 days early (again) but this time it only lasted for 2 days. If this is confusing, for example: 1st month: period starts on the 21st as it had been for the past year - 2nd month: period starts on the 14th - 3rd month: period starts on the 7th - then last period started on the 2nd of the month, 3 weeks ago... lasting only 2 days. My belly has been sore, I've felt nauceous (I have never been prone to feeling nauceous outside of pregnancy) and my breasts have been swelling and feel heavier than usual. I have a 2 year old and I know what it feels like to be pregnant, though I do acknowledge that symptoms are so similar to feeling pre-menstrual. My next period, if it does arrive, will come in a week if it is going to do the same strange thing and arrive 5-7 days earlier than the last. But what if it is late? My partner and I don't use contraception, we use the withdrawal method along with a joint decision that while we aren't exactly trying to fall pregnant, a surprise would be welcomed of course as we want to have atleast 2 children. Should I rush out and buy a pregnancy test? The suspense is starting to stress me even though I'm trying really hard not to stress. I've bought pregnancy tests in the past in a whirlwind of paranoia, only to result in feeling both foolish and dissapointed. Need lots of support and advice. Please help.