What is my SIL's problem? Please read

Nicole • Mummy of 3💙💙💗 #4 due October💗
To start with I've always felt like she doesn't like me. Ever since I got with my boyfriend (her boyfriends brother). I have no idea why I'm alot younger than her and I'm really shy I don't do anything wrong to anyone. She recently had a baby girl in July. I'm now 17 weeks pregnant with a baby boy. The other week I was walking back from a hospital appointment and saw her pass by in a car and she gave me a horrible look. I asked my boyfriend If he knows if she's got a problem with me and he said he doesn't think so. So I put it to the back of my mind and forgot about it. Now earlier today me & boyfriend was talking and he told that he thinks she has got a problem with me because she's been telling him not to get excited about the baby because it might not be his. Trying to put it into his head that our son might not be his. I haven't even slept with anyone else I have no idea where she has got this from. My boyfriend said he thinks she's jealous because while the whole family has been excited about the new baby on the way, she's been the only one being weird about it ever since she found out I was pregnant. He also said she was hoping for a boy when she was pregnant, and that he thinks she is jealous of that too. He told me that when she said this to him he remembered when I said about her giving me horrible looks and thought what actually is her problem? When he told me I couldn't help but cry because it's so unfair that my boyfriend is being told not to get excited about our own baby when I haven't even been with anyone else I know who my baby's dad is😢 I feel like she's ruined the whole experience for me now 😢 I don't understand why she's said this to him, should I do something or is it just jealousy? 😢