On Wednesday, a visit to Phoenix's doctor found nothing wrong.
"That Wednesday night I went to go make him a bottle and found a little black dot," Haylee said.
"I pulled it out and had to take a double glance because it had a body and legs."
The mum was sure the bug hadn't entered the tin after it had been opened.
"I opened the tin on Monday morning and by Monday afternoon he was sick," she said.
The formula tin said it was made in Singapore, by Wyeth Industries, a subsidiary of Nestle.
Both the Ministry for Primary Industries and Nestle confirmed they were investigating the find.
A spokesperson for the Ministry for Primary Industries confirmed it received the complaint on Wednesday night.
"The infant formula and the insect are en route to our laboratory in Auckland where analysis will be undertaken.
"We'll consider matters such as the type of insect and its origin."
The ministry couldn't comment further until the formula had been received and analysed.
Haylee said she was told on Friday afternoon the bug was an aphid, but its origin was still being investigated. The ministry confirmed this.
She said she'd also let the supermarket she'd bought it from know and they had pulled the product from the shelves. She's since changed brands.
"Since the change, I've got my old baby back. He's happy, sleeping and feeding without a fuss," she said.
A spokeswoman from Nestle said the company had been contacted by the parent on the Wednesday night and had continued to communicate with her.
"The quality and safety of our products and the health of the babies who consume them are non-negotiable priorities.
"For this reason, we have commenced an investigation to review this batch and our factory operations. This investigation will look at all records including production of this batch, testing and sampling records and consumer feedback."
The spokeswoman said they hadn't received any other complaints about the batch and was unable to comment further until the investigation was complete.
Haylee said it could have been a one-off incident.
"Just be careful. You put a lot of trust into what you feed your baby and it was the last thing I expected to find.
"I'm just more concerned there may be other children out there. It might be one-off but it might not."
The Rotorua mum said since the ordeal she was cautious about checking every bottle she made for her son.
