4mo constant straining - interfering with feeding

[pigsunited]

4mo DS is mixed fed - bf at night, and on bottles of mostly EBM during the day, but has a bottle of formula about every other day.

He usually does a poo every day, consistency is runny/seedy and a mustardy yellow colour, but with mucus in it (occasionally green). This has been the case since he was a couple of weeks old, and I have mentioned the mucus to the GP and lactation consultant, but they didn't seem particularly concerned.

In the last couple of weeks, he has started straining after taking about 2/3 of his bottle. He then seems to still be interested in the bottle, but pulls off immediately every time it is offered. He is usually sick while he is straining too. I have tried everything - massaging his tummy, bicycling his legs, pushing his legs gently up to his chest, cooled boiled water etc. (He does hold his legs very rigid though which means I can't always do these moves).

Even when he does poo, he clearly doesn't get everything out as he is still straining afterwards. If I hold him upright, he generally seems to feel better and stops straining.

I'm feeling guilty now about giving him formula as I'm worried this is what is causing the problem. However, unfortunately I can't get him to bf during the day (a myriad of problems that I have posted about before), and I don't have quite enough milk to give him only EBM. I will take him to the GP or health visitor next week, but just wondering if anyone has any thoughts in the meantime?