Thank you so much for the sweet reassurance.
I've scraped the depths of my memory and I can't remember straining or bumping the area and I've had no cuts, bites or even grazes recently to cause an infection - which is probably what's causing the most anxiety; a complete lack of even a cause to suspect. Whatever it is seems to be floating beside the muscle and can be moved a little.
I did have had a very bad cold last week so I've been entertaining that maybe my system is still sorting it out. It's also soft (which - I think is encouraging?) so it could just be an abscess to drain. Ugh I'm so worried and it's 3 in the morning and I have to study and just remembered I have to teach today good grief, it never stops disrupting does it. *curls up into a little ball with tea and a cookie and cartoons*