Four years ago we used our joint savings to put a bathroom in our home so mom could live with us. What about look-back?

I guess you don't necessarily need a Power of Attorney, but can get "signature authority" in order to access mom's account. Here's a good article on an attorney's website that talks about why joint bank accounts aren't a good idea:

If you have a joint account with your mother, you will most likely want/need to get rid of it because Medicaid will look at the money in the joint account as being all hers, which might mess up mom's Medicaid eligibility. I would suggest separating your money into two different accounts and setting up a Durable Power of Attorney, so you have access to mom's bank account to help her pay bills, without your money being mixed up with hers. This way, Medicaid can look at what is just hers.
Most state Medicaid programs do have a 5-year lookback period. However, they may have different rules regarding what the money paid for--in this case, it was a bathroom so she could stay in your home...they *may* look at that as "allowable" as it may have delayed placement into a Medicaid facility. However, these lookback rules are complicated and may vary between states, so it would be a good idea to contact your local state Medicaid office

You should research the Mediciad look back period in your state for transfers and money gifts. I thought it was 5 years, not 7. Good luck!