While they would not be able to put debts in the plan that incur after filing, your budget can include medical, and other expenses that are reasonable and necessary. You do not need to stick to your budget, and can put any savings in the bank for these 'rainy days.'
If you incur an unexepected cost, you can always petition
the court to suspend payments and /or defer payments to the end of the case, to give you immediate funds to assist, your plan can sometimes be lowered if you have greater expenses too down the line, upon motion to the court. (depends on the claims that are filed by your creditors, it sometimes frees up some room in the plan to modify it)
If you incur many bills after filing, you can also look into converting the case to Chapter 7
, whereby you can file a statement of after aquired debts, and can add them into that case as well. There are special requirements on coversions, so be sure to reveiw them with your lawyer.