take a deep breath
While I have not been in this exact situation before, I do feel that I may have some constructive advice from a married male point of view.
First and foremost, I would suggest taking an over the counter pregnancy test. There is no sense in breaking yourself down and causing yourself stress over a situation that may not even arise. Let's assume now that the test is positive; this does not truly mean you are pregnant. Schedule an appointment with a physician to confirm pregnancy.
If you are pregnant, I recommend sitting down with both Ted and Eve and openly discussing this. I recognize this is a very difficult conversation as I have had very similar difficult conversations in my past. However, you owe it to them both to be forthright with the knowledge and your feelings about it.
I do not believe it will ruin their marriage. My wife and I have discussed this very situation at great length, and while it is not what we are truly wanting, we would embrace our partner and our new child and show them the love and respect they each deserve. If their marriage is strong enough to handle polyamory, it is likely strong enough to handle any consequences that result.
Even if you are not pregnant, I recommend you still site down with Ted and Eve and discuss this topic at great length. I would bring it up more as a "what if" scenario than a "pregnancy scare" scenario. If the conversation goes well, you may feel inclined to tell them you thought you were pregnant.
I hope all goes well for you and that you find peace and happiness.