Introduce breastfeeding gradually in small dosage if your baby has been exclusively breastfeeding since it can cause coughing or reflux. Mine is doing that now. Waking up more in the night as well because of that. So just be careful and consistent.
and I knew this, I would have given my baby 1 feed of formula from early age to make her use to of it.
Oh no, I hope your little one starts tolerating it soon! 
I hope things get better. Keeping you two in my thoughts.
When I started to introduce formal to my little guy I started out with just a 1/2 oz to 1.5 oz of breastmilk. I did that for a week until I added a full oz and just increased the formula oz slowly. He did really well with it. It helped him sleep longer at night too.
I hope it gets better!