It's possible to have a tie through the centre of the tongue, but not underneath. My son is in this position, although he also had a 100% tie underneath that had to be cut twice (thin tie that turned thick at the end - the first actation consultant we saw missed the thick part at 3 days and it was cut a second time, by a paediatric consultant, at 5 weeks, and the tongue's own restriction was diagnosed at 12 weeks at the specialist bf clinic at the John Radcliffe in Oxford. One of the advisors there had had a friend, who'd co-authored her book on the latch, with the same problem).
It looks like the top of a heart, upside down. Apparently it means you have a 1% margin of error in latch technique, when most people have about 10%. I also have very big boobs when milk-producing, 34H, which can cause further problems in this situation as the sheer weight pulls the nipple from the mouth, and the baby is already having trouble milking the ducts efficiently - they use tubigrip slings at the clinic sometimes to add a bit of extra support.
I recommend seeing your GP as a starting point, as some are great. If yours isn't, contact Mervyn Griffiths' secretary at Southampton General - he is wonderful, and if your GP makes a referral he will squeeze you in asap. He is the leading international expert in tongue-tie so really we're lucky to have him in the country, and he is also lovely. When he met us, the first thing he did was greet my son, shaking his hand and saying hello.
Alternatively the bf clinic at the JR in Oxford are absolute experts, too. If you aren't even remotely local, either will be able to recommend someone who is, I imagine.
I'm going to start trying bf again this week, I had to stop as the frustration and misery of constant misdiagnosis and partial diagnosis and bad advice and pain meant I had nothing left in the tank by the time the real problem was established. There's only so long you can take pain on this scale, even without the rest of the hassle. Personally I have found expressing a lifesaver - he's still getting bm, and I have a great supply, so I can now go back to trying to sort bf with renewed vigour. If your baby is causing you a lot of irresolvable pain, for a complex physical reason, then it can be a godsend.
I would avoid seeing a lactation consultant. Non-obvious tongue-ties need very specialist treatment, and both the board certified lactation consultants I saw set us back a very great deal - weeks, in both cases. I'm sure a lc is fantastic for the more ordinary bf problem, but when enduring very painful feeding, and a very hungry baby because the milk transfer is totally inefficient, misdiagnosis and inappropriate advice is the last thing you need.