Yes you need to relax. It's only a run. Millions of people run every day. It's not a big deal. I've been running for years and it always takes me at least two miles to really warm up. It's not a biggie - it's just a bit harder for the start of a run. The problem with a 3k is that you might be in the warm up zone for all the run.
Some runs are hard. There's often no reason. Take a rest day and go again after that.