Period and PeriodIndex for multiple of frequency behave unexpectedly
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
pd.show_versions()
atnow=pd.Timestamp.now()
aaa = pd.Period(atnow, freq='3W')
print(aaa.start_time)
2018-12-24 00:00:00
aaa.start_time
Out[55]: Timestamp('2018-12-24 00:00:00')
aaa.end_time
Out[56]: Timestamp('2019-01-13 23:59:59.999999999')
bbb = pd.PeriodIndex(start=aaa,  periods=2)
bbb
Out[58]: PeriodIndex(['2018-12-24/2018-12-30', '2018-12-31/2019-01-06'], dtype='period[3W-SUN]', freq='3W-SUN')
bbb[0].start_time
Out[59]: Timestamp('2018-12-24 00:00:00')
bbb[0].end_time
Out[60]: Timestamp('2019-01-13 23:59:59.999999999')
bbb[1].start_time
Out[61]: Timestamp('2018-12-31 00:00:00')
bbb[1].end_time
Out[62]: Timestamp('2019-01-20 23:59:59.999999999')
```
#### Problem description

My original problem/example is more complicated than the simple example I shown above. In the above example, first get the time now:  atnow=pd.Timestamp.now()
Then, aaa is the 3W (3 week) period based on today.
   aaa = pd.Period(atnow, freq='3W')
Both aaa's   start and end are correct 24Dec2018 and 13Jan2019

Then, using aaa, form a PeriodIndex called bbb. I use periods=2.
bbb = pd.PeriodIndex(start=aaa,  periods=2)

Because of periods=2, I EXPECT this to be 2 of 3-week periods. So I expect the following periods:
bbb[0]  start 24Dec2018   end 13Jan2019    
bbb[1]  start 14Jan2019   end 3Feb2019

Instead I got the following observed results: 
bbb[0]  start 24Dec2018   end 13Jan2019
bbb[1]  start 31Dec2019   end 20JanFeb2019

Hence although there are 2 3-week periods, the next period is not starting where the previous period ended, which is what I expect.

I checked Github and saw 'other' issues with frequency multiplier and PeriodIndex, however none of them the same issue as I discussed, but they may be related.


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#### Expected Output

#### Output of ``pd.show_versions()``

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.15.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.23.3
pytest: 3.6.3
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.14.5
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 5.7.0
sphinx: 1.7.5
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: 1.2.1
tables: 3.4.4
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.4
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.5
lxml: 4.2.2
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.8
pymysql: None
psycopg2: 2.7.5 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
