hello lately i have been feeling discomfort on my left si...

That essentially just means your intestines are currently inflamed. Why? That depends. could be anything from something you are (an allergy or food poisoning) to things like IBD or IBS. Given you said there was “a bit” and not “a whole lot” I’m gonna say it’s most likely just some normal inflammation that’ll go away in a bit, but just keep an eye on it and if it doesn’t go away and you still have that stomach pain and mucus combo after your period is long gone maybe contact your doctor and see if they want to run any tests.