you have simply over stretched your perineum - the thin piece of skin between the vagina and the anus.
because this was a new position and I expect you went deeper than you realised too.
there's no long term damage or anything - you just need to make sure that you use plenty of lube from now on.
it'll be a tad sore for a few days and the same thing might happen the next few times until this piece of skin gets used to being stretched.
you can help things along - lay in a comfortable position and using some lube slide the tips of your thumbs (palms facing down) into the entrance of your vagina and gently stretch the skin out and downwards until you feel a slight tingle.
Gentle use your finger tips to massage the lube into this skin while it is stretched, then allow it to relax again.
repeat several times and you should find that this skin begins to stretch much easier.
This is also recommended for women nearing labour as this will help reduce the risk of having a tear or episiotomy.