Logging lithium battery discharge: why this curve?
First:

There are many reasons why your voltage would go up. As suggested by @jippie it can be heating in the boost converter in the pack. It can also just as easily be heating in your resistors, causing them to increase resistance and as such reduce drained current.

What's worse, slight imbalances between the two resistors' values will cause slight heating differences, which will cause bigger differences due to different heating, causing bigger differences in heating, etc etc, cause larger and larger measurement inaccuracy.

As @NickAlexeev suggests, if you are writing this for a mass-read publication you need to take this seriously or not do it at all. You will either mislead a large group of people with your findings or be quickly debunked by any one of "us professional EE engineers" living in your country. Best case scenario it's all wasted effort, worst case you're helping the world to yet more misunderstandings about EE and batteries and there are already way too many.

You need to:  


Keep the current controlled and constant, so you can rely on your data and know that for every single measurement you can rely on it, exactly and accurately. Research current sink circuits and if you need ask more clarification later. The best solution will be one that either doesn't get hot at all (heatsink+fan), or one that is compensated for heat. Or preferably both.
Preferably measure back that current accurate to a percent or better (which in this sense is less than one percent), since a 1% error in your current will easily build up to a decent skew in your data. But at least, if it's fixed and controlled, you can know that all your measurements will be skewed the same.
Measure the voltage with a reliable set-up, if you want you can do it with an Arduino, but not using your load as a divider. That's basically pointless. You need the voltage to be reliable as well, that goes for the one you measure and the one you measure against. Which is either a decent reference, or a very stable and reliable power supply. Most people choose the reference, but it's not unheard of to make a 0.5% stable and repeatable 5V supply for something like this, but it's harder. The Arduino (Atmel MCU) has internal references, but also a pin that you can connect a reference to. I suggest the external option, since the internal  one isn't very much more accurate out of the box than its supply voltage from a linear regulator.
Make sure you have a very accurate time-base. There are arduinos that use the internal RC oscillator of the processor: these are not in any way, shape or form accurate. You need one with a Crystal Oscillator that is capable of keeping decent time.


Once you've done all that, if your time also happens to cost something, you are going to have spent much, much more in materials, tests and research than the $199 for the battery tester that Nick linked in a comment, so you may as well go for that.

And you need to, but this is when you buy the tester as well:


Read up a lot about how these packs work (what's a boost converter? how efficient are they on average?), what the batteries do and what the risks are with low grade electronics or batteries, because for a good, or even decent, article you will need to explain this well and clearly to your readers. Plus it helps you understand what you are actually measuring. Oh, and don't forget to think about what the mAh rating means and how that relates to the boosted voltage. the "mAh" number that's printed (on usually the more crappy ones) is for the internal battery, which is rated at 3.7V, not the 5V output. That's a free-bee you get from me.