No mine didn't act any differently in the months/weeks/days leading up to the proposal. We had discussed marriage and looked at rings together so I knew he was going to propose- I just didn't know exactly when. But when he suggested a weekend getaway to the Keys then I knew he was planning to...
Avoid the chain stores and go to a local jeweler they can either work with you to customize it or can show you things that are different from the big chains.
Signs of pregnancy are pretty much the same as signs of pms and your period starting so it's hard to say. If you've had negative tests then you're likely fine. If you're really worried see a dr and ask for a blood test