I'm not sure what the gastroenterologist told you, but I'll explain the meaning of your labs and you can decide for yourself.
When a doctor orders TSH w/reflex to FT4, the order means that the lab should analyze TSH and if it's out of range, the lab should, then, go on to analyze Free T4. The reference range your lab uses is 0.4o - 4.50, which means that your 0.43 was actually within the labs range, since it's greater than 0.40 and less than 4.50. That means your results are "normal" and the lab would not analyze Free T4.
So the way it works is that TSH is a pituitary hormone that senses the amount of thyroid hormones in your blood and when thyroid hormones get low, the pituitary gland produces TSH to stimulate the thyroid to produce more thyroid hormones.
It's all counter-intuitive... typically (in a perfect world), when TSH is high, thyroid hormones are low and when TSH is low, thyroid hormones are high.
In your case, there could be one of 3 things happening:
1) Since your TSH is so low, your thyroid hormones could be really high, causing hyperthyroidism, which can cause stomach issues, tiredness, etc but also causes things like sudden weight loss, diarrhea, heart palpitations, rapid heart rate, hand tremors, etc. You don't mention any of those.
2) Your pituitary, for some reason, seems to believe that you have adequate thyroid hormones, therefore, it's not producing enough TSH, which in turn is not stimulating your thyroid. This causes another form of hypothyroidism called Central or Secondary hypothyroidism. This is when the thyroid works fine, but there's a malfunction in the pituitary gland. This is still a type of hypothyroidism and needs to be treated with replacement hormones. Common symptoms of hypothyroidism are acid reflux/GERD and other stomach issues, constipation, weight gain, hair loss, fatigue/tiredness, muscle/joint aches/pains and many others.
3) You could have the autoimmune condition called Hashimoto's and your blood work just hasn't all caught up yet. To determine if you have Hashimoto's, you'd need to have antibody tests - Thyroid Peroxidase Antibodies (TPOab) and Thyroglobulin Antibodies (TgAb).
The treatment for Hashimoto's and Central hypothyroidism will be the same - replacement thyroid hormones.
If you aren't supplementing Vitamin D, you should be doing so, as Vitamin D is necessary for the proper production and metabolism of thyroid hormones.
If you've been tested for Vitamin B-12 and Ferritin, it would be helpful if you'd also post those results, as they, too are necessary for adequate production/metabolism of thyroid hormones. If you haven't had them tested, I'd recommend you do so. Deficiency either/both of these can also cause lethargy/tiredness and B-12 deficiency can cause brain fog/inability to concentrate.
Ferritin is the iron storage hormone... Iron deficiency can also cause extreme tiredness/lethargy.
It's always best to post reference ranges with results for the best comparison since ranges vary from lab to lab. Was there anything abnormal on CMP?
As noted, a TSH by itself isn't very useful because we don't know what your actual thyroid hormones were in Feb, just as we don't know what they are now. We can see, however, that TSH is considerably lower now than it was, then... how do symptoms compare to the way you felt then?
You said you don't have palpitations or hand tremors... do you have any other symptoms of being hyper, such as weight loss, diarrhea, irritability/irrationality, rapid heart rate, etc? Or do you have any other symptoms of being hypo? Just based on the symptoms you provided, we can't tell if you might be hyper or hypo since they can both present with tiredness and stomach issues.
Have you, by any chance, had a thyroid ultrasound?