One feed at a time, and yes there is an increased risk of colic, and reflux, and constipation, and respiratory infections, and gastroenteritis, and ear infections, and diabetes, and dental malocclusion, and leukaemia, and for you, diabetes, and breast cancer. Amongst others.
Formula isn't the 'safe alternative' that most of the replies on this thread imply. It carries significant health risks, short and long term, to you and your baby.
I know that right now you are overwhelmed and stressed, and you think it's because you're breastfeeding, but the truth is the first few weeks of parenting are overwhelming and stressful even if you are formula feeding. I'm sorry if my response seems harsh but I get really annoyed by all the 'just stop, it doesn't matter!' replies. It does matter, and if you want to stop breastfeeding you deserve all the information so you can make a fully informed decision. I've encountered too many mothers who learned this stuff when it was too late and then regret switching to formula.
www.ncbi.nlm.nih.gov/pmc/articles/PMC2812877/
www.bmj.com/content/369/bmj.m875
It's important to make feeds up properly, powdered formula isn't sterile and needs to be mixed with 70° water to kill bacterial contamination, or you need to use pre-made formula.
www.nhs.uk/conditions/baby/breastfeeding-and-bottle-feeding/bottle-feeding/making-up-baby-formula/