Shareholders of BHP Billiton Limited (ASX: BHP) may be starting to get a little nervous.

After a sharp rally in the miner’s share price recently, which saw it climb from just $14.06 to a high of $19.44 just over a fortnight ago, the shares have started to slide again. They’ve fallen almost 13% since peaking and are down 3.9% today alone at $16.92.

The losses can likely be attributed to a pullback in the prices of iron ore and oil, which are BHP’s two most important commodities. Although both remain considerably higher than the levels they fell to earlier in the year, thanks to a surprise rebound in recent weeks, investors may be starting to doubt the sustainability of those rises.

Iron ore, for instance, is currently fetching US$57.87 a tonne after shedding 0.8% of its value overnight, according to The Metal Bulletin. It was fetching almost US$64 a tonne earlier in the month, however, while oil prices have also stagnated in recent weeks with many analysts doubting their ability to climb any higher.

The fact is, both commodities are still stuck in a heavily oversupplied market, with little driving growth in demand. Despite the recent rallies, this oversupply situation could drag prices lower again.

Although BHP Billiton maintains an advantage over other competitors thanks to its sheer size and low-cost operations, it is by no means immune to falling commodity prices, as we’ve seen in recent years. If these rallies do prove temporary, then BHP’s shares could fall back to the low levels seen in January, and perhaps even lower.

Shares of Fortescue Metals Group Limited (ASX: FMG) and Rio Tinto Limited (ASX: RIO) have also fallen 2.9% today, which is perhaps a reflection of the market’s waning confidence in the sector.

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Motley Fool contributor Ryan Newman has no position in any stocks mentioned. Unless otherwise noted, the author does not have a position in any stocks mentioned by the author in the comments below. You can follow Ryan on Twitter @ASXvalueinvest.

The Motley Fool Australia has no position in any of the stocks mentioned. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.