i have similar problems going, or i did until very recently. my tummy doctor put me on movacol, its a powder you add to water (or juice etc) and it stops me getting as constipated although i need to take it every day for it to work, and it took a while to take effect but i feel so much better now. my tummy pain is much lower, i dont feel all full and bloated all the time and my IBS is less annoyed because stuff is moving through my tum better (and the constipation was irritating it).
and i had no relief from the constipation when i tried senna- but i've recently found out that it often makes tummy spasms worse so i guess it just increased the spasm that was causing the constipation!
maybe it's a good idea to ask about movacol? a number of us take it and although it tastes nasty it really does work. and maybe ask to see a tummy specialist? i see a gastroenterologist at St Barts Hospital and they've greatly helped my tummy problems, and they know all about HMS and EDS and treat my tummy taking into account my EDS.
i also find it helpful to use my tens machine on my tummy and stick it on a massage setting, and manually massaging my tummy also helps a little. and stretching my back and tummy muscles sometimes works too as it stimulates my gut to work a bit better.
i hope this helps and you feel a little better soon!