You're pretty sure your hair used to be thicker than it is now but you have no idea why! I've had this experience before, for me, it was excessive use of relaxer but I've recovered from that. Here are some factors that can cause hair loss and thinning:
Excessive use of chemicals e.g. synthetic hair dyes and relaxers.
Excessive use of braids and/or weaves.
An accident, e.g. getting burnt at your hair salon.
Post-partum hair loss. During pregnancy hormonal changes may cause hair shedding to pause and hair to grow, thicker, fuller and faster. After you give birth, as your hormones rebalance it is not uncommon to see hair falling out.
Stress can lead to many adverse health effects and hair loss is one of them.
Medication - hair loss can be a side effect of many medicines. If you are taking prescription drugs ask your doctor if the drug can cause hair loss.
A medical condition, e.g. liver failure, kidney failure, lupus and skin diseases (e.g. alopecia areata). My sister incurred severe hair loss as a result of lupus but it's all grown back. We thank God.
Ageing as you get older hair will tend to get thinner, in fact, by the time you get to your 60s, 70s and beyond if you have not incurred any hair thinning you are truly blessed!
Protein deficiency - this is very unlikely to be the cause of hair loss in the developed world where most of us eat a lot more protein than we need to. Even if you don't eat meat there is plenty of protein in grains, legumes, milk and eggs.