Yes. S26 is well known that will cause constipation. I am feeding my baby S26 too. I also have this problem, but lucky just mild. If you don't want to switch the formula, I've got an advice from a midwife - just add extra 10ml water when you do the formula, remember, just 10ml ONLY.
Also, if you're introducing solid to your baby, e.g. rice cereals, don't add formula into it, add water instead. Offer water for baby to drink after each solid. Although my baby doesn't like water, but one sip is better than nothing... My friend told me I can start offer my baby fruit juice (10ml juice + 100ml water) around 7 months. May be you can try this too.