Can we have more information than that as that is nowhere near enough for us to work with as there are hundreds of reasons why you could feel faint.
When do you feel like this? Is it mainly at a particular time/ when you're doing a particular activity or can it be whenever? As said above, are you eating properly or have you got any problems regarding your behaviour towards food/ an ED such as restricting, purging (by use of laxatives or vomiting), overeating, binging etc. If so, are your doctors aware of this and are you seeking any help for this or being regularly checked out? What are your periods like and is it worse around this time? Do you have any issues that you know of with your blood pressure? Have you any heart problems or does your heart ever feel like it's missed a beat/ become irregular/ suddenly sped up or slowed down for no apparent reason? Do you have any medical conditions? Are you on any medications? If so, what and how long have you been on these? Did these episodes start at around the same times as you started the meds? How long has this been going on? Do you get out of breath easily? Finally, what do you feel like when you say you 'feel like you're going to faint' and have you ever actually fainted? And, approximately, how often does this happen (every hour, several times an hour, a couple of times a day, daily, around every other day, twice a week, weekly...)
I know it looks like a HUGE list of questions and I wouldn't expect you to have put that much detail in your first post but answering as many as you can would be a huge help for me to see what may be going on and what help you may need/ if it's potentially normal.
I wouldn't say for certain right now that you need to go to the doctor for bloods or anything because it may not be something that shows up or requires bloods and also, sometimes people can normally, every now andd again, feel faint especially with certain medications.