There are two types of consent - informed and implied. If you allowed the drip to be given, that's implied consent. You don't always need to have physically signed a consent form in order for you to have 'consented' to the treatment/intervention etc.
I do sympathise with you though and understand why you'd want to complain. After my section I wasn't allowed anything to eat or drink due to the mag sulf I was being given. I was then told I should mobilize, so had to get out of my bed and on to a chair. Unsurprisingly after no food or drink and a few different types of meds, I felt nauseous. I happened to mention it to the midwife, who proceeded to put cyclizine through one of my lines. It was all over so quickly and I was so mad that I'd been given the drug without any warning or asking if I was happy to have it (I wasn't - it was bloody obvious what was causing the nausea and nothing a few minutes sitting in the chair wouldn't have solved). I never made a complaint. To be honest, there was SO much about my experience of that hospital that I could have complained about, but I just didn't want to relive the nightmare by complaining.
Anyhow, back to your original question. If you look on your hospital's website, it should give you an address and contact for complaints. The hospital are required to respond to any complaint. Or you could go through PALS (Patient Advice and Liaison Service).