How to fix incorrect size automatically added to cart?

[MireiaP]

Hi! 

Since a couple of weeks ago we've been receiving complains from our customers, because when they try to order one size it automatically ads the smallest size to their cart. We've been trying to replicate it but it doesn't happen to us. 

Any ideas on how to solve it?

Our store is: www.4ccccees.com

many thanks in advance!

Mireia