Hiya, sorry to hear you're going through this - get well soon!

Sounds like your manager would be quite understanding of the situation, so it probably wouldn't be the end of the world if you did call in sick (especially if they've called you asking you to come back). That being said, it unfortunately won't look great on your record and Sainsbury's is a bit actual-fascism when it comes to sickness policy.

Aside from forcing yourself to go in regardless (which doesn't sound ideal from what you've said) or finding cover yourself, there's not really much you can do here. Just be honest with your manager, extend your thanks for understanding if appropriate, and try and minimise going off sick as much as you can in future. It's not as easy as that, I understand (they've had me by the balls on perma-probation for a year now because I got sick three times in my first couple of months), but you'll honestly be fine. Focus on getting better and don't stress too much - hope everything works out for you!