I am sorry that you are going through this and especially after just having a baby! I went through the same thing after having my third child and I was a mess for a while there. My fears, too, centered on brain tumors and MS. (I still worry about MS) I had all the symptoms you did, plus problems with my ears ringing really loudly, pressure on the right side of my head, headaches, ect... Plus all kinds of just strange feelings in my head. I was 100% sure I had a brain tumor and was shocked when my CT Scan came back clear.
Have you seen a doctor about the way you're feeling? They diagnosed me with Postpartum anxiety and put me on meds and I got to feeling better after that point. If you're more into trying a natural route, there are herbal supplements that you can take for anxiety as well. My sister-in-law took St. John's Wort for her PPA and swears by the stuff! I am currently taking that now for my recent bout of anxiety - again, health anxiety and again, MS fears - and it has taken the edge off of my anxiety. I don't think it's working as well as the Zoloft did, but it's getting me through this anxious time fairly well.
If you need anyone to chat with, feel free to PM on here! Hang in there and don't give up! The chances that you have a brain tumor are next to nothing and especially since you've already been checked out by your doctor!