i said yes but i said i'll think about it more once i review the formal offer.
This last bit may not have been given much attention, possibly not written down in interview notes, not communicated by the person you said it to to the other people involved in the decision process, possibly (if you kind of mumbled it out) not even something they'd honestly remember if asked directly now. Consequently, folks there may now see you as having verbally accepted and then tried for more money, which can put an employer off as at that stage they'd expect a little mutual faith and that further salary adjustments would happen after you've shown some performance on the job.
If that's happened, your best bet is to make it clear to everyone your request wasn't out of the blue. Write saying something like (but not word-for-word, people can google): "I hope there hasn't been some misunderstanding during these negotiations. You had asked me if I would accept X, and I said "yes, but I'll think about it some more once I review the formal offer". On doing so, given
(list the couple most-reasonable explanations for what in the offer made you increase your ask), I asked if you could stretch to a touch more than discussed, as I'd more more comfortable with that. Still, these are minor concerns: I have faith in my ability to demonstrate valuable contributions in the role, and in your company to tweak things when justified, and long-term win-win relationship is the most important thing from my perspective. I'm hoping to hear your thoughts, and will call at XXam/pm, but feel free to call me first on X if you prefer, or to suggest another time. Best regards, Turntables"
Can I still accept their original offer?
In a formal big-company officework kind of environment, offer letters often have an expiry date on them. If yours hasn't expired, then yes - you can still accept, though they may still back out or just start termination immediately if they really wanted to, and local laws allow. Otherwise, you have to work through this with them and they may or may not want to issue a new letter, even if for the same salary/conditions.