are you making bottles correctly?
all babies strain to poo, they go all sorts of shades of red and purple even for the sloppiest of nappies.
My midwife did recommend that if i thought little CRX was a bit bunged up that i use one carton of the premixed milk (you can get single cartons of the SMAs) as there was a slightly different formula that seemed to loosen the bowels.
If your baby is pooing every other day then i wouldnt be too concerned, but mention it to your midwife / health visitor next time you see them.