It’s just impossible to say, because of your obfuscation on the detail of the incident.
Your follow up post makes it sound like you were flung from the waltzers at a fairground!
Yet, you were at her house. And it was no-one’s fault. Garden swing, perhaps?
But as any of these details could be a lie so you’re not “outing”, who can possibly say?
You say that your MIL wasn’t there to witness it, and you laughed it off when she then saw you. Your baby cried from the shock - but “someone” took your baby who presumably had calmed down by the time the someone took them to MIL. Afterall, you wouldn’t have let them take him out of your sight whilst you got your breath back, calmed down, and made sure no permanent damage had been done.
If that’s actually what happened, and not part of your detail change, how on earth was MIL supposed to know how bad it was?
Here’s my best guess... you were on a garden swing which tipped too far backwards (as you landed on your back / head) and you tumbled off it. So - sudden, frightening, but not a great height. It could have been serious, but it wasn’t. It still bloody hurt though, and because of your baby, it has understandably shaken you up with the “what if”. But MIL only saw you up and about after, with baby fine, and you brushing it off. So it’s hardly surprising she thought it was a minor incident!