This weekend I got a DUI. The way the cop stopped me seemed weird to me though. He said he pulled me over for speeding. I knew I wasn't speeding because I saw him behind me and when I asked how fast I was going he said 66 mph!!! He immediately started asking me questions about how much I had to drink that night and before I knew it the cop had me out of the car and standing on one foot and blowing into a breathalyzer. How can I get pulled over for speeding when I was going SLOWER than the rest of traffic and end up with a DUI??? It seemed like the cop was looking for a reason to pull me over because he followed me for a while before he pulled me over but I know I didn't swerve and he even said I didn't when I asked. Can he use any reason to pull you over and then investigate you for something else??
First off, I hope that you invoked your Miranda rights and did not answer any police questions or perform any field sobriety tests, although from your question, it sounds like you did the latter. Unfortunately for you, what the officer did was perfectly legal, although it hasn't always been that way. In fact, prior to several key US Supreme Court decisions in the past two decades, such stops were found to violate the Fourth Amendment; but not anymore!
As long as the officer had probable cause to believe you committed a crime, which in your case was a Vehicle Code violation for speeding (however slight), he or she is free to pull you over and investigate any unrelated criminal activity. Whether it was the officer's subjective intent to give you a ticket for speeding or to find any reason to pull you over to investigate you for DUI, as long as there is probable cause to believe a crime has been committed, the officer's subjective intent is irrelevant.
In my experience, it is not at all that uncommon for DUI investigations to stem from the most minor Vehicle Code violations. What is even more unsettling, even if an officer pulls you over for violating the "wrong" statute, the stop does not violate the Fourth Amendment, so long as there is a "right" statute that applies to your conduct. That is to say, if an officer pulls you over for speeding, but it turns out that you were not in fact speeding, the stop is still legal so long as there is another basis for the stop, such as having a broken tail lamp. The lesson to take away from this is that you can be pulled over for the smallest of infractions as a pretext for a collateral criminal investigation and that officers have a great deal of leeway in justifying such stops. You've been warned!