I was on a walk

Well, it could actually be a doorway of some sort. Often, places in nature that seem or feel magical, are. Perhaps a nature spirit guards that place. As for the stone, I'm not sure.
What kind of stone was it?

that weak feeling was your inner power. appearently the elemental that was on the other side of that "doorway" wasnt particularly happy with you being there. not saying you shouldnt go back, but this stone you had, if it was a Quartz you may want to charge it first as much as possible, and then wait 24 hours and go again, meditate once you get there, offer it to expose its self to you in your mind and feel its presence. and see what happens xD

For Druids, atleast ones in the Ovate class anyway, they can communicate with trees by placing their left hand on the tree and their right hand on the ground (for grounding). I'm not quite sure why you felt weak, but maybe the next time try placing your right hand on the ground as well and see if that helps. The branches may have been a doorway just like it appeared, maybe to a place where some Otherworld type beings live. Perhaps, they want to try to communicate with you first to see if they want you in their territory.

I could also direct you to some videos online that talk about Druidry and about how to communicate with trees, if you'd like. They're really good videos and the teacher is really knowledgable.

thanks that really helps I think it was an amber crystal so I should charge it before I go there again. And it only happens when I am alone

I think you encountered what I call a ward. It's basically a magical protection charm that simply stops whatever it's meant to. I put up a simple ward around myself all the time to keep bugs away. The feeling you got was probably the ward doing it's job. The second time must have been a second line of defense just in case the uneasiness doesn't do it.

ok but I am going there again right now so what should I do?

all right I went back there about a hour ago and there was a dead tree and then five trees around it and the five trees seemed to be in the position of a pentacle.

That's some powerful elemental mojo you got right there. Something happened with the dead tree, but that's a powerful sign to behold. Try and figure out what type of tree it was. Though first I would lay down a few offerings for the spirits in that place (at the doorway) and don't go in. Honey is a great one, try and get as natural honey as possible. Really any sweets are good for it, go on Halloween and see what you feel.

thanks I marked the tress with pentacles just to be sure But now I don;t know what to do?

When it comes to wooded areas, they are generally worked in by other magickal practioners. The place you found may be a site for a coven. If you feel the trees crossing over is where a portal lies, its best to walk around it, not through it. You won't enter into any place because portals are spiritual, but walking through such energy can leave you feeling odd, sick, or just plainly strange because your energy mixes with it.

If you feel welcome in the area, then it may be a great place for a ritual or spell, but if you do not feel welcome, then it's best to keep walking and leave it be. You don't want to mess with such things when you don't know the nature of them.

I've seen many such places. In the woods where I used to live near, by the Desplains River, there were many. One in particular that I will never forget was a circle about 10-15 yards wide created by dead trees. In the center the ground was burnt (a bond fire I assume). Many times these aren't natural creations and were specifically positioned for ritual work.