"therefore I agreed only for them to test it for that and nothing else."
Any documents you signed would detail what you consented to.
"This wasn't an emergency situation or eles I would've went to a hospital instead of a sort of walk in clinic."
Hmmm. You left that out of your original post.
"It's just wrong to tell a person one thing,they agree to it and then do something behind their back without their consent or knowledge!"
The law doesn't provide a remedy for all wrongs. Primarily, there needs to be an injury for the law to provide a remedy, normally.
"Random drug testing is done to inmates"
No it isn't. Inmates still possess some rights against search and seizure. Additionally, the government is restricted against that sort of thing, but not private parties, normally. If I get a chunk of your hair I can have it tested for whatever I want.