Be very careful!
A fairly common error when picking sorrel is to pick Lords & Ladies instead. This is the small mace of bright red berries that arrive in mid-late summer. Every bit of this plant is poisonous, with the leaves acting as a strong irritant.
L&L leaves are very similar in shape to sorrel, although the wings at the back of the arrow-head shape tend to be more rounded, while sorrel has sharp backward pointing tips. Also, L&L tends to be slightly more glossy with a balloon-rubber-like feel in the more mature leaves - Sorrel is more succulant. At this time of year, look out for a green spike sticking up in the middle of the plant, which is the spadix (male organ) of the L&L, surrounded by it's leaf sheath - it's rather fallic if you peel the surrounding leaf away.
Finally, L&L tends to grow in/near woodland, whereas common sorrel tends to grow in rough-ish pasture near the shelter of dips and hedges.
Common sorrel may well be peppery/sharp, but WILL NOT hurt your toung. It sounds like you have made the above mistake.
I reccommend cross-referencing with several id-ing books before eating anything from the wild.
River Cottage Hedgerow Handbook is good as it helpfully points out dangerous look-alikes.
Good luck
Mark