My 18 day old has been on a combination of breastfeeding and formula top-ups since birth due to low birth weight. For the last two days she has been waking every 1.5 hours screaming but being really fussy with feeding. She has also been straining and arching her back like she has a lot of gas, but burping has limited results. Despite a few settled times (usually after offering a formula feed for 30 minutes) she has been really unsettled. She used to breastfeed 15-20 minutes but now it's gone down to 10 minutes then she pulls away and refuses more. After a bit of burping/repositioning she'll latch back on but only comfort suck then sleep. Sometimes she'll bring up milk but usually after a nice long feed. Now today she hasn't done a poo (usually a nice wet one after each feed). Is all this straining because of constipation? Is the constipation due to formula?