I had some very weird dreams whilst pregnant too, Emma. I have pretty vivid and sometimes weird dreams anyway, but I had some strange pregnancy-related ones.
I suspect that it is the subconscious's way of processing all the information we are bombarded with during pregnancy, along with the worries and concerns that niggle at us.
When I am anxious about something, I have dreams where things are going wrong for me, and I can't make them go right - and often the thing that is going wrong doesn't actually have much to do with what I am actually worrying about - for example, I will dream I am back at university and it is half way through the term, but I don't yet have my time table for all my lectures, so I have missed lots of them. Or I am trying to ring someone, but cannot dial the number without making a mistake.
So I would guess that you aren't actually worried that the baby is someone else's, but that you are naturally concerned and worried in the same way that most of us are at one time or another whilst pregnant (it is a huge responsibility to be growing a whole new human being, and knowing that you will be responsible for raising them too), and your subconscious has added that to something you've seen on TV to produce a dream that is helping you process your anxieties.
Congratulations on your pregnancy - my dses are 17, 19 and 21, and the lovely, squidgy baby stage seems an awfully long time ago! You have got so many wonderful things to look forward to, and I wish you a happy, healthy pregnancy, a stress free birth and happy motherhood!