My infant daughter has this too. I read on NEJM that bath/hair/body products containing lavender or tea tree oil/fragrance can cause the breast buds as they have estrogenic effect. I have eliminated these recently. I would suggest you do so too.
This is usually called premature thelarche, all babies go through what we call minipuberty near birth and in girls there is a surge of some hormones from the brain that then go away and some can develop a little response that shows as breast buds, these usually resolve due to lack of ongoing pubertal levels of hormones and by age 3 this should be gone, however if this is enlarged compared to prior exam then workup may be needed, firstly to measure the brain hormones that stimulate the ovaries to make estrogen, next a pelvic ultrasound via the tummy area to look for a simple cyst(a common problem and not usually requiring treatment other than monitoring) or even a case of rare hypothyroidism-the brain tells the thyroid gland to work via a hormone called tsh, when the gland is underactive the tsh rises and this cross reacts with the puberty hormone receptors showing up as early puberty, rare but important,
so if any labs are needed I would get LH, FSH, estradiol, free T4, TSH, if the FSH is elevated but LH is not this is a benign process usually and is the minipuberty response and can be monitored, but ultimately this may need to be assessed and interpreted by an endocrinologist, so talk with your doc, if they are unsure ask for a consultation for peace of mind.