I had a period either the end of October or beginning of November. I had sex on November 28th. Then my next period came the 23 of December. I have irregular periods I never know if it will be every month or every other month. For over the past week I've been super tired, way more than usual. No nausea. My back has been aching. I've felt twinges or spasms in my stomach off and on. Is it possible that I'm pregnant? If so, how far along?

Was your Dec period normal? Have you had sex after your Dec period?
But no, by the way I read and understand your post, you would not be pregnant,
1. had sex before last period
2. no sex after last period.

I mean there is a very very slight chance but most likely this is just a period coming on, or you have perhaps the flu or viurs

My Dec period was heavier than normal. No flu or virus that I'm aware.