The natural acid in cider is malic acid. To get a feel for what it tastes like buy some "sours" candy, its a sour tasting acid which is used for a food acid in candy and the like.
if this is the flavour it's natural, you can get rid of it by doing a malo-lactic fermantation, or wait for the flavours to balance better with age. If it is vinegar it will have a distinctive odour and you may see an acetobacter 'bloom' on the surface. This will only happen if o2 gets in.
Generally the advice is to relax and wait.