Green poo a little slimy exclusively breastfed


My little girl is 3 weeks today and the past 2 days all of her dirty nappies have been green and sometimes slimy. It started 2 nights ago just before an episode of spitting up to the point where she couldn't catch her breath (it was very scary). It happened again 10 minutes later so I called the doctor and they weren't concerned... just said if it happens again to call back. Since then all of her poops have been green and a little slimy. She has also been very gassy both ends and always appears uncomfortable when she's trying to sleep. Could this be an allergy? I haven't eaten anything different.. is it possible for a baby to develop an allergy after 3 weeks of no symptoms? I'm also worried that she has a tummy bug :( if this persists I will be brining her to the doctor but just wondering has anyone else experienced this? Do you think this is normal? My first child never experienced this so I'm a little worried