Low iron levels are one complication of Crohn's that you need to worry about and be diligent to counteract. Low iron levels result from the body not taking in enough iron, normally due ti malnutrition or because of inflammation or after surgery due to short bowel syndrome.
I have had problems with low iron for many years. There are the following strategies that you can take:
1. Oral iron supplements: by far the most convenient and non invasive method - the problem is you need to find supplements that you can take without getting into trouble. For me those are Tardyferon (a French brand) and - if used with 3 times the amount of water - dissolvable tabs, namely Loesferron. All of them have 80mg in iron(II) each. You can only get them with a prescription, the stuff you get without has iron content of 3-5mg which is virtually nothing.
2. Iron injection, basically a "2 minute" infusion for those who can take it. You need about 4-5 every week to get ferritin up from 14 to a healthy 80 or 100. Beware, this can attack your venes and can cause other side effects, plus it normally means that you are back at 10-20 ferritin after 3-4 months, if nothing else changes. However, as an initial "getting iron up again" measure it makes sense.
3. Iron infusion, for those who have the time to sit down 2 hours per week for 4-5 weeks. Basically the same as 2.
4. Blood transfusion - this is an emergency measures if, as a result of low iron levels, your hemoglobin levels have dropped to dangerous, life threatening levels. I had them three times in my life, each time was a wake up call, but only after the third time did I really act to get iron levels straightened out.
5. Lastly, but that is hard to do, a normal diet and not flaring and being in general remission will help with iron levels.
5A. Extra tip, once things are back on track, sport and supplements (magnesium, zinc, pottassium, B6, B12, D etc.) really help for both Crohn's and iron levels. Plus an apple a day.
My advice would be to go for 1. On a permanent level, but for immediate results ask for iron injections or infusions. Don't take this lightly. The exhaustion (and being pale?) is a sign the low iron levels already resulted in anaemia, that is low hemoglobin.
Lastly, I don't think ferritin is something that can fluctuate due to inflammation, having had a few dozen blood tests over the years including ferritin, I know it is a very stable figure that really only gets influenced by iron supplements, infusions etc. I think Catherine was thinking about the short term iron levels (which is named "iron" on a blood test), they can jump widely up and down due to what You just did a day ago or that very morning.