Hi The Hiding, and thanks for your questions. I think Fire Dance pretty much answered the deja vu question.The Normal
As to the song lyrics in your head, it may not be anything paranormal. It could be that you heard it at some point previously --on the radio of a car passing by, within a movie score, on a TV show, etc. Subconsciously it may have registered in your memory, though consciously you weren't even aware you were listening to it. Then later on, it seemed to pop into your head out of no where, then keep playing and playing. This is a pretty common occurrence for most of us. My "turn around time" for such songs is about 3 days. I'll have the weirdest song in my head and I'll have to think and think and think, where did I hear that one? Then, I'll remember it may have been playing on a website I was surfing, or something random like that. It's amazing what the subconscious mind is aware of, and absorbs.The Paranormal
Regarding your friend's mom's comment, if the song (or even a phrase, word, image, or smell, etc.) stuck in your head is in fact a legitimate message of some sort, in my experience not only could it be from an angel (and not necessarily your "guardian angel" either), but it could also be communication from discarnate humans ("dead" doesn't sound right to me so I don't use it, since if they're still communicating, obviously they're not really dead), or it could be from any of a variety of other spirit entities, as there are many
out there. Here's a story psychic medium John Holland shared in one of his newsletters. Getting things stuck in his head is a normal part of his work, if not his daily life in general, lol: John Holland's Snoopy Incident
They attempt to communicate with incarnate humans like us at any given moment, to anyone who might hear them. Some people are sensitive to this, some not as much. It's like the high-frequency cell phone ringtones that only teenagers can hear and adults can't hear (which my kids, by the way, really get a kick out of testing on me --I cannot, for the life of me, hear those things).
So since you're a "sensitive," you may hear things. This ability, clairaudience, is probably only one among other "-claires" you experience, which might tie into why you think you're experiencing deja vu. You're picking up psychic information empathically, still you are not yet able to piece together these messages and their meanings. This will come with time as you get older as a person and experience more of life, and as you develop your psychic abilities, which takes time and work.
Exercise Yourself --Wisely
Have fun with your abilities for now. If you're one of the fortunate few growing up in an atmosphere where you can openly discuss psychic topics and ask questions, this presents a great opportunity. Buy a book on "psychic development," with psychic exercises in them. Choose a book that really jumps out at you ("resonates" would be the proper word for that, lol). Then, do the exercises with whomever has an open mind and will work with you, preferably your mom if you're a minor, or someone else whom Mom approves of you working with (caution: very clever pervs like to prey on people who are searching for answers in life, so always let your loved ones know what you're doing with this stuff, OK?) If you're not in a such an environment, search for some exercises on this website. I've seen some. It's a pretty well-monitored site, though the caution still applies, even here --as you exercise your abilities, also exercise common sense.
While you're having fun, you will also be sharpening your natural skills, thus better understanding yourself, and helping you enjoy life even more. Keep a journal, record your experiences. Feel free to share your psychic development stories with us in the future if you'd like to!