Both my jobs I work on my feet all day with clients.
Tell your manager, and if you ask them not to share the news, they can't ,however, I felt telling everyone fairly early meant when I did feel awful, every understood.
As an employer they need to do a risk assessment with you asap. Its legal for them as soon as they know. You can request in there to wear comfortable shoes (if you dont have already, I have to wear dolly pumps and now wear trainers), and they should be giving you at least a 20 min break per 3 hours on your feet.
I found with faintness (which i had a lot of) the only thing to help was going out the front door and sitting outside on the floor if need be!
It can't be stopped, however you need to sit asap and somewhere cool.
Be drinking enough water, and I was also advised be eating almost constantly, I usually had a packet of grapes with me to be chewing on or light biscuits. Also had a bottle of Coke to sip on in between water through the day for a sugar rush. This helped with faint times.
Because you rent the chair, Im thinking surely you can work as you please? Slow down your bookings, take enough breaks in between to sit or eat?
Im now 6 months pregnant and have found since around 4 months ive been comfortable again, I do not get faint or feel sick.
My problem now is sore feet, sore back and tiredness!!
You'll get past this bit quickly enough hopefully, just be sensible. You know your body, so dont push it x