If your lump persists, you should get it checked out by a doctor who can take a more detailed history and physical exam. Oftentimes these lumps are reactive/secondary to infection and/or inflammation and spontaneously resolve. If the lump does not resolve, your doctor may choose to order additional imaging tests such as ultrasound.
Hello and welcome to the forum. The good news is that you are 13 years old so not the typical demographic or age that will need to worry about breast cancer. The rate of cancer for girls between the ages of 15 to 19 is .2 per 100,000. For younger than 15, the risk is even lower. But it is always concerning to find a lump. Please speak to your mom about it and you can have a doctor confirm if further follow up is needed. One thought is that these are what are called 'breast buds' and are going to develop into part of a more mature breast. Hormones are involved. If it is squishy, then it could possibly be a cyst which is very common. Here is an older thread where one of our medical doctors believes the hard lump under the nipple is probably a breast bud of another 13 year old girl. https://www.medhelp.org/posts/Breast-Cancer/i-have-a-hard-lump-under-my-right-nipple-and-im-13-what-is-is/show/673464