My mare is exactly the same! But i have been told many times that a horse wont eat something that is bad for it out of choice and also that there are also two types of straw
1) barley straw- this is soft fluffy and usually very yellow/golden - this is relatively harmless to horses and used to be fed to them back in the dark ages as forrage!
2) wheat straw - This is usually darker in colour and much crispier (if that makes sense, almost stick like) altogether harder and hardly fluffy at all. This again doesnt usually harm them in small amounts and unless your horse is prone to getting bunged up or bouts of colic it really wont hurt.
The only inconvenience is the loss of bed and having to top it up with new bales more frequently. She (if not a colic sufferer) will do just fine on straw while she has her baby.....if she doesnt supprise you and do it in the field on her own. hehe
Pics of the foal when it comes pls!!!! All the best!