I agree with the others, what you have have does not look like San Pedro. Although the flesh of the ribs tends to shrink in when their sick, and they lose their colour etc, the thorns are much bigger on a pachanoi or peruvianus.
My firend only really figured it out when his flowered. Although he moved it into a 'better' spot when it did and he said it just went black and fell off. So maybe summer is better, because they flower and they are dormant in winter anyway.