I am with my second agent now and I feel like her interest is waning because my book hasn't sold yet. I have a feeling she might drop me soon. I've been a busy writer in the meantime though, and I have written two more books. If she were to drop me and I were to query the other books (which have not been subbed), should I mention that I have had two agents in the past? Also, my agent has had my second book for over a month now. How long should I wait before sending her a nudge about it?
Wow, what a slacker.
First, you're projecting all your anxieties on to your agent (she's going to drop me, she's lost interest) and I'm going to guess that if she's anything like the rest of us, she's just damn busy right now.
Second, you also don't mention how much time your book has been on submission. Publishing is glacial most of the time. I've sold books that have been on submission for YEARS.
Third, if you've had two agents already, you're a terrible risk for anyone else absent some grievous form of Bad Agent Behavior (losing interest doesn't even register here) and most likely seen as a difficult and troublesome client.
Talk to your agent. Ask her for a reasonable idea of when she'll read your work and understand that things can get in the way of meeting that deadline.
As long as she's still taking your calls and answering your emails you need to suck it up and keep writing.