Need help with sodium

Had gastric bypass 2 and half years ago . lost 157 lbs doing good until blood pressure went way down and was put in hospital twice for being dehydrated. For some crazy reason my body will not hold fluid.The doctor put me on meds to help with this problem but they are just not working.I drink and go straight to the bathroom.when ever I get a sinus infection like I had this week I go into dehydration and blood pressure fall even lower. normal blood pressure for me is about 97 over 67. on Wednesday when my son took me to the doctors office my blood pressure was 89 over 62 I just knew we were on our way back to the hospital. lucky for me I was not quite dehydrated yet and was able to get myself back.if any one has any ideas please let me know.
I will see all my friends at the next support group meeting.

Thanks