I've been pumping occasionally since my baby was about a month old. I use a manual Medela pump and have always been able to pump enough for one feeding. It's been a couple of weeks since I last pumped and this morning I tried and tried but couldn't get anything more than a few drops. I tried relaxing to help letdown but that didn't work. I need to leave my baby for a few hours this afternoon and don't know what to do if I don't manage to pump a bottle. Can I use formula? Are there any negative repercussions if I do this? My baby is 4 months old and exclusively breast fed.