I lied at MEPS. Will they go back and look through my medical records?

You're kinda dumb for asking this with your real name posted. If you had forgot something you were told at 6, it's forgivable for forgetting. You just admitted in writing that isn't true. Consider deleting this.

It's a good thing you are thinking of this now vs after you are sworn in, as when they do find out you lied you would have gotten an other than honorable discharge for a fraudulent enlistment.

I concur with others who say you are on a public website. Nonetheless, it’s out. Not sure when you lied, or on what documents you lied on, or to whom you lied. This I know. If the military doctors find a problem during your entrance physical that prohibits you joining military service, they will not let you in. This happened to me. I had high blood pressure. They found it and sent me home from MEPS. But two months later they called me back to MEPS, rans tests and daily BP checks. Eventually, I passed those tests and got in to finish a 20 year career. I like your current integrity. If you get in, do right and do well. If you don’t get in, lift you head high for nothing shall be impossible for you!

They will now

No big deal dude! I got drafted, went to flight school, went to Nam came back to Ft. Sam, got my annual flight physical, and was told I had a "heart murmur" that I would grow out of. Every physical/EKG I've had since then (Army and VA) has said the same thing. I'm 70 years old now and wonder how old I gotta get before I "grow out of it"! Oh, did I mention I spent 22 years in the Army, 16 of which was on flight status? Want to get a "shrink eval"? Wait until you have 19 years in service and volunteer to go on jump status! My CO thought it a great idea but the Doc thought I was nuts! Go for it!

Regardless of whether you told them or not, they're sending you to a cardiologist to be evaluated. So they read something in your records or heard a murmur in your physical exam. If the cardiologist recommends you be allowed to enter, you're good. They are the specialist for that diagnosis. Most murmors are benign in young people so don't stress it too much.
Source- Army PA

I was a recruiter and was surprised at how many people have or had heart murmurs. Seems they check at meps.

Hipa laws prevent them from looking at your past medical records.

Well,you were free and clear until you went public!!

I lied about my heart murmur 45 years ago. They found it after I passed out in the mess hall after working for 18 hours. Navy MD asked me if I wanted to go home. I told him I couldn't. My WW2 Veteran Dad would be freaked out. He said "you can stay". I did.
Didn't keep me out of Marine Infantry. Nor did my very flat feet.