today was my first day back at work-1week post op and I had the exact same experience. I hadnt eaten any food, only my protein drinks as usual. Dont know what caused it but I only made it 4 hrs before I had to leave work and go home to rest.
You probably are dehydrated. That happens to me every now and then and then I try to get more water in. Take it slow at work and try to sip all day. If its hard to drink from the water bottle or cup, drink with a straw. Straws have been very helpful for me.